One of the main problems with bass is that the human ear is 'tuned' to hear mid sounds. As volumes decrease the bass is perceived to drop away very rapidly. (There is a name for this phenomenon can't remember what it is, but there's lots of literature about it out there. RobinH will know what it's called - nice to see you back here BTW Robin
). The most obvious solution to this is to boost the bass in your mix, but this can end up smothering it completely. Listening to a lot of bass in 'pro' recordings the thing I have noticed is how unbassy they are, really low mid sounds that you kind of 'know' are supposed to be bass - it's only when you turn up the volume that the real bass becomes apparent. Using eq and compression can help with this, here are some tips on eq that I use (experiment though, find out what works for your mix).
Is your bass boomy, taking everything over? Try pulling down some gain at around 50Hz.
Need more bass clarity, particularly at low volumes? Try boosting at about 400Hz.
To give that kind of 'middy' bass sound I was talking about earlier (the one that fools you that you are hearing bass) try boosting at about 800Hz.
To give even more 'cutting' power to your bass, boost at 1.5kHz - especially good for plucked bass sounds.
Some stuff to try out anyway. Good luck,
KasioRoks