Bass is the main instrument that when soloed can sound great--how you want your bass to sound--but in the mix can get lost. You turn up, gets boomy. I have to say that this has as much to do with your other instruments. You have competing frequencies, they add up, phase out, etc.. One thing to do is to cut the very low freqs on your guitars, keys whatever. Leave a good thump and crack on the drums. In the EQ of your mastering 2-track out, dump some of those lows from the overall mix. What happens to the bass when you cut the boom is it loses volume. But now you can push it up a bit more and it will be somewhat tighter. BUT YOU SAY YOUR GUITARS SOUND GREAT WITH THAT THICK LOW-MID! Ok, do you want a song with just these great guitars, or a song with drums, bass, guitars, keys, shaker, theramin, .....