i tink two subs are better than one for many reasons - better room integration & lower distortion at any given spl are certainly prime consideration. but, even at low x-over points, i tink the use of two subs, set up for soundstage, prowides better imaging, than only one. there must be ambient cues in the recordings that account for this, as it seems using one sub always compromises soundstaging, ime. even at low x-over point, some of the bass always seems to come from wherever the sub is. the best compromise for only one sub, for soundstaging, is centered directly between the speakers, imo. nearfield is even better. but, this may not always be best for bass response, aesthetics, etc.
john mentions my subs, flanking my monitors. ya this is how i like it - best soundstaging that way, for sure. but, i do not get room-shaking bass response w/them in this location. (well, except for a coupla recordings!

) part of this is due to the large room size, and the fact that the room opens up to other equally large spaces. but, i'm sure i could sacrifice natural soundstaging for room-shake, if i moved my subs agains walls... in my much smaller bedroom, a pair of floor-standers, rated -2db at 28hz, easily energizes the room, much more than the main rig, due to speaker location & smaller room size...
and tyson, tho i agree w/vmps being the best music subs at anything even remotely resembling a sane price, i tink it's due to their simple engineering design (ie: big boxes) more than use of exotic materials. this allows for lower distortion, lower frequency response, & higher output w/o need of equalization and/or extremely expensive exotic hi-excursion drivers needed when trying to squeeze low frequencies out of a small box. when i 1st got my larger subs, there was no carbon fibre or rubber surrounds. when i upgraded to megawoofers, there was an improvement for sure, but it was wery subtle. and, in fact, the 15" "megawoofers" are still w/foam surround, & poly cone - yust bigger magnets...
doug s.
btw, a pair of vmps larger subs, a quality outboard x-over & s/s amp will not cost any more than a pair of aci titans.

diy on the subs w/pre-made cabinets, or build yer own cabinets & save even more... of course, for most rooms a pair of vmps original subs would be more than sufficient.