Nice idea with the sub, not going to go to sub freq tho

But an easy way if doing it will be to build a passive sub filter, can be done with few resistors & caps.

The sub will be an addon to you N900 right?, it looks to be to big to have in the N900 (mugen cover).

The dock i made gives more bass then stock N900, you could try the other speakers first with the small 2x3W amp.

of course to make an optimal system you need 2x amps, 1 for speakers 1 for sub, of course this amp cannot operate in bridge or parallel (i haven't tried).