Not to be a nay sayer, but I think the replies above have mostly failed to home in on the real issue here:
 
 
First of all:

Jeffiphone
And here's the newbie question...... If all tracks and master are registering around-12dB, shouldn't the volume be consistent between all tracks, and the mix be leveled correctly?

 
No, absolutely not. Some elements of a mix will be louder than others, and some elements will seem louder or quieter in a subjective sense, regardless of what the meters say. The meters cannot tell you how to mix. They're there to help you stop clipping things, and make other technical assessments.
 

I've read a little on dB vs volume/loudness, but I think I'm missing something here. Yes, my monitors are not the top of the line, and my room is not treated, but the discrepancy is too striking, so I must be doing something wrong.

 
Well, the thing you're doing wrong is as above. Stop trying to mix via meters and trust your ears more. But that brings me to the point about monitors and room.
 
The listening environment is NOT a minor factor. It is in fact, the most important factor in the whole thing. You would seriously be better off mixing in mono with a single good monitor in a decent room, using only the bundled plug ins in the entry level version of any DAW, than you are working in a bad room.
 
And the outcome of listening environment problems is in fact exactly what you've described. There is some combination of room characteristics that means by the time the sound hits your ears, the frequency balance is all askew. So you're not really hearing a neutral enough representation of your mix to create one that travels well to other rooms, or your car, or wherever.

The biggest problems are in the low end, and putting it simply, the overwhelmingly main barrier to a good mix is a room that either boosts or cancels out significant low end. You're clearly in the latter situation, so a low end that seems right in your room is way too boomy elsewhere.

Is there a magic plugin that can tell me the solution? 👍🏻

Sadly no. To mix something properly, you have to be able to hear it properly. So you need to improve your listening environment.

How big is your room, and how is it laid out?
 
 
 

To add to my previous post and the others.  Don't use the visual feedback as the main criteria for your decisions.
 
Let your ears do the work, not your eyes.  If it has too much bass, turn it down, not enough, turn it up.  Or contour it so it fits better.  It can sound like too much because the space isn't carved out.

John T
 
The listening environment is NOT a minor factor. It is in fact, the most important factor in the whole thing. You would seriously be better off mixing in mono with a single good monitor in a decent room, using only the bundled plug ins in the entry level version of any DAW, than you are working in a bad room.
 
And the outcome of listening environment problems is in fact exactly what you've described. There is some combination of room characteristics that means by the time the sound hits your ears, the frequency balance is all askew. So you're not really hearing a neutral enough representation of your mix to create one that travels well to other rooms, or your car, or wherever.

The biggest problems are in the low end, and putting it simply, the overwhelmingly main barrier to a good mix is a room that either boosts or cancels out significant low end. You're clearly in the latter situation, so a low end that seems right in your room is way too boomy elsewhere.

Is there a magic plugin that can tell me the solution? 👍🏻

Sadly no. To mix something properly, you have to be able to hear it properly. So you need to improve your listening environment.

How big is your room, and how is it laid out?
 
 
 

Well the OP mentioend using headphones as well as monitors.  
 
Headphones remove the environment from the equation.
 
I agree the Room can make a difference (sometimes really big) when listening on monitors.  But a lot of the time it is really more about using the gear properly.
 
Near field monitors at reasonable volumes negate a lot of room issues.  Ok, negate isn't the right word, but it does minimize them.  Most people make the rookie mistake of mixing at volumes much louder than you would actually listen to the music on an average day.  These volumes cause the sound to bring out the worst in the room.  Even a small bed room can be fine with reasonable listening volume, and enough "stuff" in the room to kill/breakup the waves.  At least to the point where you can get a good feel for reasonable balance.  

Does the resulting CD sound OK elsewhere?  If it sounds OK on your PC and if you put the tunes on your phone or in another car and it is OK, the issue is your car.  Some cars have "Bass Enhancement" or loudness enhancement that attempts to give people the home environment on the road.  I personally don't like all the extra bass this puts in.
Just my 2 cents...

Brian Walton
 
Well the OP mentioend using headphones as well as monitors.  
 
Headphones remove the environment from the equation.
 

 
Not really. Headphones are another environment, with issues of their own. Definitely worth checking the mix on headphones, and indeed alternative sets of speakers of various types, but they're not a silver bullet for this problem.
 

I agree the Room can make a difference (sometimes really big) when listening on monitors.  But a lot of the time it is really more about using the gear properly.

 
 
Sorry, but I could not disagree more. If you can't hear what your mix is doing in a reasonably neutral way, then you can't make mixes that translate well outside of that room. There's no way round that.
 

Near field monitors at reasonable volumes negate a lot of room issues.  Ok, negate isn't the right word, but it does minimize them.  Most people make the rookie mistake of mixing at volumes much louder than you would actually listen to the music on an average day.  These volumes cause the sound to bring out the worst in the room.  Even a small bed room can be fine with reasonable listening volume, and enough "stuff" in the room to kill/breakup the waves.  At least to the point where you can get a good feel for reasonable balance.  

I agree with a lot of that. Also, even in a good room, mixing loud is not a reliable way to work, due to the Fletcher-Munson curve; frequency response of the human ear gets flatter at louder volumes. Without getting into the nitty gritty of it, which you don't really need to, mixes that sound good quiet generally sound good loud too. Whereas mixes that sound good loud may or may not sound good quiet.

patm300e
Does the resulting CD sound OK elsewhere?  If it sounds OK on your PC and if you put the tunes on your phone or in another car and it is OK, the issue is your car. 

 
That's a really hasty conclusion to come to. The job of mixing is to make mixes that sound good in all kinds of non-optimal playback situations.
 
 
 
 
 
 

I have learned i really nice "trick" from Grammy Winner Jacquire King.
He wrote this in an article years ago. Its about getting your bass allways to the right volume. Dosent matter where you mix (other room other sound...).
 
I personally use this in every mix i do. 
 
In short its seting up your mix bus like this.
 
MY mixBass for example is  
EQ plugin
Mix Bus Compressor
EQ plugin
Tape emulation (vu meter) 
 
The trick now is that a VU meter is really sensitiv to low frequencies not to high. So if i start a mix i start with my kick drum volumen- It should peaki at -3dbvu (tape emulation VU Meter).
Things to know if you double an instrument the dbvu goes up by 3dbvu. So bass and kick have nearly the same frequencies. They are in the same ballpark. 
So my kick peaks around -3dbvu. If i bring the bass up -  bass+kick should peak at 0 dbvu.
Now build your mix around this gainstaging. And your bass should be allways in the ball park.
 
There was a youtube video horrible long but he explains it. I ll search it and post the link here.
 
EDIT
www.youtube.com/watch?v=lF2oNsiZWjc
There you go horrible long but worth looking expecially the bass volume trick ist woth this 50 minutes ;)

Man, I really appreciate everyone's help on this! Again, this forum is truly the best around. 
 
So many good ideas here that I think will really help me, and others in my position. I've finally got a mix that's been gain staged correctly, unlike my previous attempts. But it always seems to be the freaking low end that becomes my nemesis. I'm not really trying to master anything yet, but just wanna get a decent rough mix that proves I'm on the right track. I'll definitely try the song on other devices, instead of my car. I think that may be an issue. One good thing I did notice though......the "mud" factor has been significantly lessened on this mix compared to previous ones. So at least my EQ'ing is getting better (thank you HPF). 
 
I am using my ears, not just the meters. Actually, I'm using both complimentary, I think. So I'll keep at it and report back. I am gonna buy some acoustical treatment for my room this week. So I think that will be a big help.
 
Thanks again guys.
 
~Jeff

Plenty to chew on so far, especially tlw's excellent comments. As you can see, it's all a bit more complicated than it seems it should be - a common theme in this arena, in which so many aspects are unexpectedly counter- or non-intuitive. 
 
Given the good advice already offered above, I'll add just one practical observation: your car is a terrible place to judge mixes.
 
I don't care whether you have a high-end aftermarket stereo system or a stock factory installation in your car. It's too small a space, too many odd reflections, and too many resonant cavities to allow an honest representation of a recording. Even an expensive system will play tricks on you, usually adding an unadvertised low-frequency boost in an attempt to counter the acoustical limitations of a car's interior.
 
Add to that the fact that you are not monitoring bass accurately in your studio and you have a deadly combination that's guaranteed to bring endless frustration. The most common symptom will be excessive bass when you listen in the car. You'll go back to the studio and try to fix it, but the only way it sounds OK in the car is to make it sound thin and nasty in the studio.
 
I burned through a LOT of CDs chasing my tail that way, until I took my mixes to a professional studio and judged them there. That's where I was struck by an epiphany: my studio monitors were lying, my living room stereo was lying, and my car's stereo was lying. They were all lying to me, but each in a different way, making it impossible to create a mix that sounded good on all three.
 
The ultimate solution was to create a neutral monitoring environment in the studio. That meant a serious investment in quality speakers, acoustic treatments, and objective measurements to identify where my monitoring weaknesses lay. Only then could I rely on what I was hearing.
 
Mixes still sound a bit boomy in the car, but I now know that's the fault of the car, not my mix. With my ears better attuned, I now realize that even well-made commercial CDs are a bit boomy in the car. But they fall within an acceptable range of boominess because the mastering engineer had tuned the mix to a neutral reference.
 
You certainly don't want to hear that the solution to your dilemma is spending thousands of dollars and weeks studying up on acoustics. Unfortunately, that's where you're ultimately headed if you're serious about this stuff. The good news is there are things you can do now that cost nothing other than your time.
 
Your friend in this endeavor is the spectrum analyzer. There is an excellent one called Voxengo SPAN that may be downloaded for free. Start there.
 
With SPAN on your master bus, rip some tracks from your favorite commercial CDs, import them into SONAR and examine their spectral breakdowns. You will quickly see commonalities between well-made records in the way frequencies are distributed. This doesn't mean you should necessarily force your mixes to mimic them exactly, but they will give you a guide so you'll at least know when you're inside or outside the ballpark. 
 
Now, old hands will always say "trust your ears" and "don't rely on visual aids". Ignore them. At this point you cannot trust your ears and you cannot trust your monitoring environment. Visual aids are your only option. At least, until you've got a couple extra grand in your pocket.

You have to use both your ears and your meters. The problem is every studio (space)  sounds different. Especially in loe frequencies. 
In my studio the bass my sound small while the same volume setting on the same bass sounds huge in your studio. Finding the right  bass volume is definatly a realy hard task. Thats why i am so glad about the dbvu trick. It is not a rule but with this method you can be sure you are in the ballpark.
 
And this little trick works very well with the mentioned headphones. Headphones are even worse mixing bass than monitors. Littel exapmple. Ever heard the Bass on some AKG 701 and than some for example beyerdynamics or even beats? XD Mixing bass on headphone by ears is a task that cant be accomplished as long as you dont use your headphoens for years and you know them really really well.