Which component upgrade will produce more BASS?

I wpuld try moving them out to the middle of the room but close to the side walls, toed in quite a bit and move your listening position closer to the rear wall. You can then adjust your bass and imaging by moving backwards and forwards, back for more bass and more centerfill on multimiked recordings and forward for less boom and more envelopement on stereo miked recordings. I know I'm mixing issues but that's what you'll find. You'll be amazed at the added depth and you can totally taylor your bass response and room nodes with your listening position.

ditto Doug's suggestions, and also some clarification on my above post:

when I say to tailor bass and imaging by moving backwards and forwards, I mean moving your listening position, in case that wasn't clear.