Since: Apr 08, 2004
First guess is that the room is not tweaked, adjusted, etc. to accurately convey the musical content.
For instance; the room has some nodes that reflect 280hz REALLY WELL. So you'll be hearing A LOT of 280hz in your mix, so you compensate by dropping the EQ on that freq till it sounds good in your room.
Then, in every other setting, the 280hz area is very weak.
Now add to this; dips, hot spots, standing waves, resonances, etc. of the room you're in, and you can see that there's a ton of possibilities for problems.
To remedy this could be quite involved; room treatment, or quite easy; moving the speakers and your listening position. Most times it's a bit of the latter, and a lot of the former.
Massive Master (username) on here has some great stuff about treating your room. Ethan Whiner has some stuff that's helpful, about bass traps, etc. Some on here have made up some DIY bass traps and had some good results.
I believe it would be a diminishing returns; the first few things do a lot of good, then each next addition may be more involved, but return a smaller improvement.
So budget, room constraints, expectations, etc. come into play.
Another angle to look at is the mic placement, and room you're tracking in (if you're using a MIC). Those both are critical to good sound out the speaker, and both issues usually can't really be fixed 'in the mix'.
Read up on some of Massive Master's posts, as he's been quite verbose on ideas on tracking, mixing, mixing expectations, getting the mix to sound right, preparing for mastering, and mastering. Good stuff that.