i definitely recommend *a pair* of subs for audio, even if ewe have full-range main speakers. as mentioned before, subs can take the load off of the full-range speakers' woofers, cleaning them up considerably. other adwantages are allowing ewe to place the *main* speakers optimally for imaging/soundstaging, not worrying about having to optomise bass response. when i 1st got my subs - a pair of vmps larger subs, my *monitors* were thiel 3.5's, which are 20h-19khz, +/-2db. the improvement in sound was *not* subtle.
ok, these are my sub *preferences* - my opinions, of course! ;~) i *greatly* prefer passive subs, such as my vmps', used w/an active outboard x-over, such as my marchand xm-9 deluxe, and your choice of decent-quality amplification. 1st, i believe the quality of the amplification & x-over electronics is *far* superior to that offered in active, powered subs. also, the flexibility is far greater, both for initial set-up, & for future changes. also, i don't like the amps & electronics being so close to (inside?) the subs. also, most powered subs are smaller, using electronics to get lo-frequency response. passive subs (like vmps) use no trick electronics, yust simple engineering w/large, appropriately-sized boxes. yes, my subs, @~18d-26w-39h, *are* big...
then, there's the cost-thing. being a cheapskate, i rarely buy audio equipment that's not used, or a close-out. however, i bought my subs & x-over brand-gnu. these, w/my used adcom gfa555's to drive them (well, one was gnu, but it cost me $450, back in '85!), plus decent tara-labs cabling, cost me <$2.4k. and, the only subs i've ever heard that equal the low-end output, at such accurate, tight, non-distorted levels, were the 7' towers of the old infinity irs-lll's, now approxomated by the top-line $135k/pair genesis speakers. there's *no way* i could afford, even used, to get the quality & quantity of bass my sub system is capable of, at anywhere *near* the price, imho.
ymmv, doug s.
