Arvida, I don't work in Audition, so I can't address your problem specifically. I suspect the problem may not be Audition-specific, perhaps something you're doing 'wrong' systemically....
Can you listen to your non-rendered-but-mixed work in real time, perhaps play it out of your system into a CDr, then A/B it to what you've bounced/rendered into a stereo wav file? That might give you some clues.
I'd like to hear what you're talking about as well, so maybe you can post a clip.
Although it COULD simply be a level issue (I'm assuming you've normallized the entire track after you mixed it?), perhaps you've got some other wierd stuff going on. The loss of bass is puzzling...perhaps a comp/limiter set too high, or being affected by other things in the mix process?
What resolution are your individual tracks? (16/44, 16/48, 24/44, 24/48, etc?) and what resolution are you bouncing/rendering to?
Are you using any dither as well? Several things can affect the quality of your mix, and from what you describe, it sounds like you're way-disappointed with the results - results that are drastically different from what went IN to the mix? Audition may have a dialog box that allows you to include or exclude some of your effects and processing. Make sure these are all selected as well, before you render/bounce.
There may also be level adjustments (I've seen this in Adobe and other programs) that will deliver your mix at a higher or lower level than when you started - kind've a pre-normalizing function. (-6, -12, -18, etc.) If that's the case with Audition (although I doubt it), you may have to poke around in your settings and correct this as well.
It could also be a case of where in the chain you're putting certain effects. That "no-bass" thing would sure seem like a big clue to what's (not) going on in the mix.
Good luck....post a clip if you can, too.