For some reason I have run into the mind-set that Kevin has seen - a perception that having more than one sub is more trouble than it's worth and that they have a fixation on low frequency response flatness. News flash! The low frequency response in any real world listening room is going to be frought with significant peaks and dips in response and they will be different depending on where you listen. Depending on the room, two subs can even these out or make them worse. The only way is to evaluate your room and if it has the potential to be a "problem room" - for instance if it is a perfect cube, don't even bother with one sub! If the room has more "ideal" ratios of height x width x depth and/or if it is irregular in shape, two subs can help with flattening the response.

All I can say in conclusion is that firstly, my room is more "ideal" than not - it was purpose-built to be a listening room. Secondly, each time I double the number of active subs - going from one to two and from two to four - the low end gets more subjectively "solid" (and yes, I am compensating for the volume increase with the subwoofer's amplifier). So unquestionably - in my room each doubling of the number of subs presents a very audible benefit in subjective quality - the response measures and sounds flatter and the extension of the low frequencies (playing my organ demo CD for instance) is undoubtedly improved. The lowest notes get obviously more authoritative.