I figured it out - there is an "avoid clipping" toggle which prevents the volume slider from going all the way up. 
 

Interesting thought, mixing in mono, Craig!  I assume many of your tracks are stereo and just summed??? Much of today's vst's are stereo, with lots of imaging and movement going on... So you really think this helps in those cases?  Most of my sound field generators are heavy in the movement realm...
 
I'll give it a try!
 
I agree with Sharke on the low end shelving... I do this a lot.  Also, carving out mids for guitar... then there's the ole' sidechaining bass/bass drum so they're not wackin' full on at the same time.  

I also mix in mono at first. I have a little Auratone clone that I constantly check the mix on, too, even after I have moved to mixing in stereo.
 
For mixing the bass I also use a great trick I picked up from Danny Danzi.  I monitor through the Auratone, and start playback with the bass right down.  I gradually increase the level until I can hear it, and then back it off a little.  This has improved my mixes no end.  I always tended to mix the bass too loud, but now it sits much better in the mix.

I guess when I said "how do you know which is right," I was asking the original poster why he thought his mixes were wrong.  I hope that with good near-fields and no extreme room issues, he can get pretty close.  Car stereos are notoriously weird-sounding and usually bass-heavy.  And, as Craig hints, a lot of "music" headphones are made to hype the bass, while generic phone headphones are optimized for voice (if, indeed, they are optimized for anything).
 
I agree 100% with listening in different environments, but changing your room, monitors, or EQ based on that is risky.  Hopefully you can do some testing of your room.  Luckily, that is no longer extremely expensive or difficult.
 

DO you use your hi and low pass filters on EVERY track?   You need to roll off all wasted low end and even high end where it is not needed.  Headphones usually do not produce the same lowend as speakers in a room.    This was an issue I had when was still more of a novice.    Most things like guitars and vocals and keys don;t need anything below 80 - 100 hz.  You need to listen but as a rule hi pass all that stuff.   Bass and Kick can also be hi passed but most like 40 or 50 hz.  use a 12 - 24 db slope.   This will clean up you mixes incredibly.  The more you use this technique the better you will understand it and properly tune each track.  All the build up of lowend cuases mixes to sound aweful.    Highend too but not as obvious.

The low E on a bass guitar is about 41 hz.  Most six-string guitars have little musical information under 80-100 hz.  I normally give the kick the power in the 40-50 hz band and cut it in the 250-300 band so the kick and bass have separate spaces.  Some do it the other way and let the bass take the low end.  If the bass and kick are both occupying the space below 100 hz the mix will be too bottom heavy and you wont have headroom for your mix.  Too much in the 250-300 region sounds muddy.
 
So as stxx said, HPF is very important to keep things under control.

Vastman
Go to sonarworks and pick up their headphone calibration system.  They have modeled a long list of headphones and their system works wonderfully on my KRK 8400's... you have many options, as you'll see once you arrive at their site.

 
haha for near $600 I think I'll figure it for myself!

+ 1 on reference track -  I really like them as an aid.  
 
I might add that I try not to do any mixing just after tracking.  My ears seem to be colored by what I think I am hearing, a few hours away kind of helps take care of it.  
 
I tend to use both earphones AND reference monitors - the later with and without a sub bass speaker.  I try to avoid 'bringing up the faders and start tweeking' - to me, it helps to isolate each important track first, make it sound the best I can,  and then fit the tracks together so they can breath a bit.  By adding tracks one at a time, I can hear when I have saturated any area of the spectrum - then I deal with it.    
 
Mixing at low verses high SPL's can attribute to a bass heavy mix.  
 
Sometimes, when the low frequency spectrum is too rich,  I will use a bass enhancer, like the two that Waves has - I can add a bit to the higher frequencies, back off on the low frequency - and still get the musical point out well.  
 
I also have learned that,  If my bass is too high, then I usually have a problem in other areas of the spectrum too - thats me.  
 
One last thing - there are tools to help in a rich mix - Melda has some very nice visual freeware, and one of my last go to's for checking the mix out is the panipulator.  I have caught more than a few frequency overload - rich problems with these little hoober doobers.  
 
Best regards,
 
LL

joden

Vastman
Go to sonarworks and pick up their headphone calibration system.  They have modeled a long list of headphones and their system works wonderfully on my KRK 8400's... you have many options, as you'll see once you arrive at their site.

 
haha for near $600 I think I'll figure it for myself!

What on earth are you talking about? The basic headphone calibration plugin is $69. 

...now this sounds like a good way to mix. Please post a (trusted) tutorial link.

for this reason I start off mixes in mono so the instruments will "collide" with each other as much as possible.