I used to own 3.5s and the issue was never insufficient bass. Getting smooth bass response was always the problem for me - probably because of room matching issues. I eventually moved on from the 3.5s. In my current room, the SMS/Rythmik sub/MMG combo provides much smoother response below 100hz than I was ever able to wring out of the 3.5s stand alone. I'm not suggesting that this is relevant to your situation - different room & different speaker - just my experience.
Marty
Marty