Good thought CATRON. Just make sure that you match the impedance from that mixer to the main mix.
Traywig,
yea its prertty much trial and error. I can guarantee that it is the acoutical space that you are mixing in. Whether a studio, or control room, if it isn't acoustically tuned with traps and diffusors, then you'll run into those problems. Take a perfect mix, then play it in your car and you're like, "what happened to the mix i just did?" It sounds like in your mixing space you have a roll off in your upper frequencies, so you basically compensate to make up for it. Then taking that same mix into a space that is a bit better controlled, the upper frequencies over kill.
What I say is, try to help the room out (is it symetrical?) or just keep it in mind when mixing.
Oh about the mono vs stereo thing, it makes a difference cause mixing mono, you have all the frequencies competing to play out of 1 channel vs stereo when you can create space between various instruments as you wish. Creating this space also puts less stress on your output, therefore less likelihood of distortion.