I've been thinking about upgrading from an internal audio interface to USB, but I can see a few options and I'd like to get some opinions, if you guys don't mind...
 
I have a Mackie 402-VLZ3 mixer which, if I don't have to replace it, will save me a few bucks. But every USB interface I've seen has its own pre-amps which raises some questions for me.

1. Running the output of the mixer into pre-amps on the USB interface will be redundant, but will it be a problem? How will it affect the quality of the signal that finally ends up in Sonar?
2. Is it better to replace the 402-VLZ3 with some kind of USB mixer/audio interface and thus get everything all in one?
3. Is there a good-quality bare-bones USB interface I could use with my mixer that wouldn't have pre-amps?

Any advice/information would be very much appreciated.

I have a 1604 and route hardware synths and guitars through it into a Presonus Audiobox 44VSL. I haven't noticed any issues with the signal being adversely affected by the preamps on the Audiobox.
 
Regards,
Dan
 

Dual preamps (some external device plus the built in preamps in a recording interface unit), as noted is not a problem, I do it all the time.  But, most of the current USB interfaces probably have better preamps than the Mackie.  In fact you may not even need the Mackie as you can connect instruments or mic's direct to the USB interface. 
 
 

dcumpian
I have a 1604 and route hardware synths and guitars through it into a Presonus Audiobox 44VSL. I haven't noticed any issues with the signal being adversely affected by the preamps on the Audiobox.

Thanks, Dan.

fireberd
But, most of the current USB interfaces probably have better preamps than the Mackie.  In fact you may not even need the Mackie as you can connect instruments or mic's direct to the USB interface. 
 

Good points. Even the current generation of Mackie mixers use their Onyx pre-amps and--I'm assuming--are better quality.
 
I just hate to spend more than I absolutely have to. For instance, I know some companies make really pared-down interfaces that can be got for under $50. With pre-amps already in place in the mixer, if I'm just going out of the mixer mains to the USB interface's inputs, how much degradation will there be?

The thing to keep in mind with audio interfaces is your not just buying hardware, most important is the software,, drivers. Without good drivers the device is a door stop. You will not get good drivers for $60. 
Just look for an interface with stereo (a pair of)  line inputs. This will either be as a channel 3/4 or with Combi jacks on the front. Avoid interfaces that have 1 mike and 1 instrument jack. 
I run a Mackie Mix 8 into my interfaces  rear panel line inputs and have also used my front panel combi jacks on my Scarlett 6i6. Both work without issues if you toggle the correct pads and switches. The important thing is to match impedance. 
I would be looking at the Tascam for my next interface. Even though AT thinks the pre amps are all the same I do think think Tascam has upped the anti. And Mackies pre amps are still top notch too. 

Cactus Music
The thing to keep in mind with audio interfaces is your not just buying hardware, most important is the software,, drivers. Without good drivers the device is a door stop. You will not get good drivers for $60. 
Just look for a interface with stereo line inputs. This will either be as a channel 3/4 or with Combi jacks on the front. Avoid interfaces that have 1 mike and 1 instrument jack. 
I run a Mackie Mix 8 into my rear panel line ins and have also used my front panel combi jacks on my Scarlett 6i6. Both work without issues if you toggle the correct pads and switches. 
I would be looking at the Tascam for my next interface. Even though AT thinks the pre amps are all the same I do think think Tascam has upped the anti. And MAckies pre amps are still top notch too.

Thanks, Mr. Music... or may I call you Cactus? :)
Good point about cheap drivers.
 
I've been looking at the Scarlett 2i2, but I'll have a look at the Tascam stuff now as well.

I used a 1402VLZ for quite a while hooked up to an M-Audio Delta 66 PCI card and it turned in good results.

Most, maybe all, interfaces use a limited range of AD/DA convertor chips. For example, I understand thst the RME UFX and the Presonus Firestudio use the same PCM4104 chip. So at the conversion level interfaces should, in theory, have a similar performance.

Though in practice they don't quite because quite a lot depends on the circuitry that supports the convertor.

The big difference between interfaces is at the analogue circuitry level. Lower noise cleaner preamps, signal processing on the analogue side of the input and output and so on. Similar to the reasons one preamp costs £50 and another £1,000 really.

The good news for your purposes is that the weakest point in cheaper interfaces tends to be the mic pres, but for your setup you'd only need to be using the line level inputs and outputs so the mix pres are irrelevant.

Another difference between interfaces is the internal mixer they carry, the level of control over it and latency. Again, if you're using a mixer which can send individual channels to the interface via the Aux3/4 bus and has the interface outputs plugged into a mixer channel so there's no need to use Sonar's input echo function for monitoring that's not a problem.

How much degredation you'd get from the really cheap interfaces isn't easy to tell without testing them. Personally I'd look a little more up-market at some of the Focusrite range or maybe Roland with two or four line inputs and two or four line outputs. They'll have some mic pres, but they might come in handy (especially if one is high impedance for guitar/bass use) and will be decent quality. Whether they'll be equal to or better than the Mackie is hard to say.

One thing to watch if replacing the mixer is the number of inputs and outputs available on interfaces, and once you're mixer-less using input echo in the DAW for monitoring becomes more important if you need to hear effects while tracking because you'll no longer have the mixer's number of inserts and aux channels.

To completely get rid of a mixer I ended up with the RME/Focusrite setup in my sig. because of the number of ins and outs I require. Not a cheap way to go it has to be said.

Thanks, tlw. That confirms some stuff I had wondered about.
 
A few months back, I was almost ready to buy a Focusrite 2i2 and then read a bunch of horror stories about getting the drivers working properly. I also read that the 2i2 is in some way inferior to the other models in the Scarlett line (don't remember the details, unfortunately).
 
But if I'm only using the line-level inputs (which, now that you point out, I should have realized [duh]) I'm wondering now if that difference in quality between the 2i2 and (say) the 2i4 will matter. Anyone know?
 

Has anyone used the Mackie Onyx Blackjack with Sonar?