I use an entry-level hybrid tube pre-amp and sometimes the sonar tube plug-in, as well. The great difference is: It is way easier to mix an already rich-sounding track than "Fixing it in the mix".
At first, it is hard to hear the difference. but after a couple of tracks over a longer period of time you will get it: A tube worns out, it changes its behaviour while recording - sometimes I think it is even the wheather. Your tracks get a unique color you will not get by using an emulation.
Second, the sound is always different compared to recordings from other people. Even if they bought the same amp.
In Short: If you want to have a unique, signature Sound than it is very helpful to have (analog) gear like a tube amp. If it is just the harmonic distortion: Save the Money by using the great plug-ins you can get today. 

The problem here is that many here are putting the tube Mic Pre right at the top of the ladder and thinking there is nothing better which is wrong.  The other issue is most of you are guitarists recording rock music so the genre is limited.  The points here are based on that a little too much.
 
Any tube Mic Pre introduces distortion by the use of valves and transformers.  If you were to do a square wave analysis on a tube Mic Pre you would be a bit shocked.  It would suck.  (as reel to reel tape also suffers bad under certain tests!)
 
So what is the alternative.  A super clean transistor front end that has way less noise, distortion and so on and would probably eat a tube Mic Pre for breakfast.  And be seriously more accurate.
 
Many engineers recording classical music or many other genres would more than likely not use a tube Mic Pre in reality.  Even recording digital sythesisers may not benefit from going through a tube mic pre on the way in. Or drums for that matter. (A class A valve circuit is fast and will handle a fast transient but a transformer is inferior at transients because of the slower rise times. So drums may sound better through non transformer circuits. Transformers have to be involved with valve circuits usually)
 
So the truth is you have a choice of Mic Pres before going into your DAW and maybe you should think about what it is you are recording and what style and whether it is in fact the right choice.  Everything you track through a tube Mic Pre will have that sound.  You may not want it.
 
Then there is the issue of can a recording that has been done through a clean Mic Pre be made to sound like a tube Mic Pre.  Well then I guess what is being said here is probably true and the result may not be quite the same but it may still sound nice after processing though if you use the right tools.  If they can emulate tape sound then there must be a decent emulation of a tube circuit out there too.  (if they can emulate the huge sound of the finest analog synths and they can, then a tube circuit should be a snap)
 
That gets us back to this.  Even if the result is nice but not quite the same if the performance is stellar than it matters not a squat.  Think of recording Frank Gambale doing an amazing solo and recording him either through a tube Mic pre or not.  It sort of becomes moot because what he plays is going to drop your jaw the same amount just the same either way.  Isn't that what is important. 
 
Maybe save your money and instead of spending $5000 on a tube Mic Pre there may be better ways to spend the money in your studio.  If you can afford it the ideal situation is to have both a tube mic pre and a super clean accurate precision Mic Pre and then have the option of both.  You will end up using both about the same amount.  All the best studios I have worked in have both available.
 
Just another point of view.

Additional to Jeffs comment: I think you speak about some low budget pre amps. if you are willing to spend ~250$ (I suspect) you will get a nice preamp compared to the one in your interface.
There are options like hybrid pre-amps, which are working as solid state pre-amps combined with a tube (example: Studio Projects VBT-1). In my opinion they are a large improvement for a low Investment and you can do both: clean and messy tube recordings - Jeff is right: Most time the signal will be messed up compared to modern pre-amps. But it is like speaking to a Death Metal guitarist: They don't want a brilliant Signal - they spend thousands to get amps messing up their signal and buying cabinets with the worst frequency spectrum you can imagine. But that is what defines their specific sound...  
 

Jeff Evans
The problem here is that many here are putting the tube Mic Pre right at the top of the ladder and thinking there is nothing better which is wrong.  The other issue is most of you are guitarists recording rock music so the genre is limited.  The points here are based on that a little too much.
 
Any tube Mic Pre introduces distortion by the use of valves and transformers.  If you were to do a square wave analysis on a tube Mic Pre you would be a bit shocked.  It would suck.  (as reel to reel tape also suffers bad under certain tests!)
 
So what is the alternative.  A super clean transistor front end that has way less noise, distortion and so on and would probably eat a tube Mic Pre for breakfast.  And be seriously more accurate.
 
Many engineers recording classical music or many other genres would more than likely not use a tube Mic Pre in reality.  Even recording digital sythesisers may not benefit from going through a tube mic pre on the way in. Or drums for that matter. (A class A valve circuit is fast and will handle a fast transient but a transformer is inferior at transients because of the slower rise times. So drums may sound better through non transformer circuits. Transformers have to be involved with valve circuits usually)
 
So the truth is you have a choice of Mic Pres before going into your DAW and maybe you should think about what it is you are recording and what style and whether it is in fact the right choice.  Everything you track through a tube Mic Pre will have that sound.  You may not want it.
 
Then there is the issue of can a recording that has been done through a clean Mic Pre be made to sound like a tube Mic Pre.  Well then I guess what is being said here is probably true and the result may not be quite the same but it may still sound nice after processing though if you use the right tools.  If they can emulate tape sound then there must be a decent emulation of a tube circuit out there too.  (if they can emulate the huge sound of the finest analog synths and they can, then a tube circuit should be a snap)
 
That gets us back to this.  Even if the result is nice but not quite the same if the performance is stellar than it matters not a squat.  Think of recording Frank Gambale doing an amazing solo and recording him either through a tube Mic pre or not.  It sort of becomes moot because what he plays is going to drop your jaw the same amount just the same either way.  Isn't that what is important. 
 
Maybe save your money and instead of spending $5000 on a tube Mic Pre there may be better ways to spend the money in your studio.  If you can afford it the ideal situation is to have both a tube mic pre and a super clean accurate precision Mic Pre and then have the option of both.  You will end up using both about the same amount.  All the best studios I have worked in have both available.
 
Just another point of view.

Good points Jeff. For my studio, the UR44 mic pres are more than adequate and I do get good mic recordings of voice, guitars, and percussion. 
I had a Alesis stereo tube preamp that I tried for about a year on stuff and it now has died. I liked it on bass guitar the most. It was interesting on live guitars and vocal but never became a go to piece. 
 
The other night, I recorded a voice and used the Presonus VT1 on the input stage just for fun. It sounded great to me. That's when I started thinking about replacing the cheap Alesis or not. 
At this point, I am not going to bother with such a cheap unit and instead, will be learning all I can about VST tube emulators. 
 
I wanted to hear from the pro's what you guys suggest and how you do it. I love the conversation and find it very rewarding. 
 
If I owned a good tube mic pre, I would definately compare with and without to my digital mic pres just to understand the true difference. My gut tells me that when all plugs are applied and the song is finally mixed, there may not be a huge difference in the sound of the finished vocal either way. Using compressors, limiters, EQ, EFX, and how the vocalist sang all play a huge part in the final product. 
 
Thanks again.

I don't have the Presonus VT1 actually and I am a full Presonus Studio One user and all. I should get it really. I have heard good things about it as well. Apparently it does sound rather close to their valve Pre.
 
The concept that vdd has mentioned is also a good one and that is the idea of having a tube pre and a high quality solid state stage in the one unit sounds rather interesting to me. And the fact you can blend these things too would only add to the sonic variations available.  Personally I might be starting there to see what is out there.
 
I have a Yamaha digital mixer with very clean and useable Mic pres on board and the thing is I have recorded everything for the last ten years or so through that and never had any complaints really. By the time as you say when you start adding some nice emulators here and there I have always managed to get the tracks sounding very nice in the end.
 
I don't dispute what Bats says either as recording through a killer valve Pre on a great slightly distorted guitar sound can be just stellar. I have done it and love it for sure! And yes bass seems to reap the benefits too. Vocals can go either way I have found. It depends on the Mic/singer combination first I feel and then the Pre next in vocal tracks.

Rimshot
I totally get that there is no replacement or substitute for good hardware on this issue. 
Since I don't own any to compare to, I am looking for those like bat that do have them.
 
I appreciate all the feedback so far. It would be great to hear a sample of with and without a tube pre and a post recording VST tube EFX applied to really understand the huge difference. I have been searching YouTube for 2 days and have not found anything yet. 
 

Now that we have aux's and patch points which allow us to record through a plugin might be a way to test just how these plugins really fair. I wonder if the sound would be different on the way in than it is when putting it on an already recorded track.

Jimmy, I'm in a session right now and having a coffee break, but will stop back and give you my take. Just for a quick answer, Jeff shares my view. The tube pre usage depends on the instrument and situation in my opinion and just adds some coloration. I personally can get similar coloration with vst effects. Not exact as hardware pres with real tubes color differently. That said, it is in the ears of the beholder as to whether or not it's "better".
 
I personally stay away from tube pres unless I am going for something specific. It's rare I use them because my uad stuff comes close enough to me to where I can process after and get really convincing results. That said, I've not heard any plugs that I have tried that come close to uad as far as saturation goes. No one has it down other than them....and trust me, I've tried quite a few. Digital distortion is not the same as saturation drive. Uad doesn't give those types of artifacts which is why they are more convincing to me. Hmm...maybe I covered it all. There was more I wanted to say but can't remember.
 
Bottom line....a great tube pre will give you the sound of different, not necessarily better. It's all subjective and another tool in your tool box. Some guys swear by them, other guys like me feel they are more hype and cost than they are worth. Unless there is a specific processing warmth or characteristic within a tube pre that makes a huge difference, there are ways to come close. You have to weigh whether or not that 2% difference only an engineer would hear justifies the purchase. Engineers don't buy records as much as the ordinary folks that don't know jack about this field. I say save your money or get a uad Rig and have several choices instead of just one hardware unit. 
 
-Danny

Mosvalve

Rimshot
I totally get that there is no replacement or substitute for good hardware on this issue. 
Since I don't own any to compare to, I am looking for those like bat that do have them.
 
I appreciate all the feedback so far. It would be great to hear a sample of with and without a tube pre and a post recording VST tube EFX applied to really understand the huge difference. I have been searching YouTube for 2 days and have not found anything yet. 
 

Now that we have aux's and patch points which allow us to record through a plugin might be a way to test just how these plugins really fair. I wonder if the sound would be different on the way in than it is when putting it on an already recorded track.

That's one of my questions. In my test, it seemed to work well putting at tube compressor on the way in. Now I want to test both ways and see if it really does make a difference. I have read that it can't but I want to test it.

Danny Danzi
Jimmy, I'm in a session right now and having a coffee break, but will stop back and give you my take. Just for a quick answer, Jeff shares my view. The tube pre usage depends on the instrument and situation in my opinion and just adds some coloration. I personally can get similar coloration with vst effects. Not exact as hardware pres with real tubes color differently. That said, it is in the ears of the beholder as to whether or not it's "better".
 
I personally stay away from tube pres unless I am going for something specific. It's rare I use them because my uad stuff comes close enough to me to where I can process after and get really convincing results. That said, I've not heard any plugs that I have tried that come close to uad as far as saturation goes. No one has it down other than them....and trust me, I've tried quite a few. Digital distortion is not the same as saturation drive. Uad doesn't give those types of artifacts which is why they are more convincing to me. Hmm...maybe I covered it all. There was more I wanted to say but can't remember.
 
Bottom line....a great tube pre will give you the sound of different, not necessarily better. It's all subjective and another tool in your tool box. Some guys swear by them, other guys like me feel they are more hype and cost than they are worth. Unless there is a specific processing warmth or characteristic within a tube pre that makes a huge difference, there are ways to come close. You have to weigh whether or not that 2% difference only an engineer would hear justifies the purchase. Engineers don't buy records as much as the ordinary folks that don't know jack about this field. I say save your money or get a uad Rig and have several choices instead of just one hardware unit. 
 
-Danny

Thanks for your input Danny. It's good to know that you are having great results with uad. I don't know much about that line at all and will research it. 
You said it well about mic pres being another tool in the tool box. That makes sense. I like the several choices vs. one hardware unit approach as well. Heck, from my single room studio in Central California, it wouldn't make sense to invest a lot in hardware at this point in my life. I can dig those that do. I would love to have better mics, outboard gear and all but have to justify it.
 
Thanks again. 
 

Jeff Evans
 
I don't have the Presonus VT1 actually and I am a full Presonus Studio One user and all. I should get it really. I have heard good things about it as well. Apparently it does sound rather close to their valve Pre.

You should get it, I got the channel strip collection on release, and they are both very good, more than hold their own against any other channel strip in my collection, love 'em