musical sub

I need to improve my system with a compatible sub for my system as it has evolved so far. My primary concern is the musicality, although Home Theatre is important, it is not as important as the music enjoyment. We watch a lot of music dvd's and play lots and lots of CD's, and of course third is the home theatre.

For reference, the main pieces of my system so far are:

McCormack HT5 amp (5 channel)
Anthem D2 processor w/ Arc
B&W Nautilus series 804 fronts, HT2 ctr, SCM surrounds
McCormack UDP1 disk player (the upgraded model)
Dennon 3800BDI blu ray player
etc.

What I am looking for is something that will add to the system, is very musical, and that will survive eventual component upgrades as I learn more and refine the sound.

Any suggestions on the sub? (or equipment)

Thanks