Two subs of any given quality level will perform better than one.

The biggest thing that helps in this regard is mutual coupling between the subs. Essentially, assuming the subs are considerably less than a half wavelength's distance of each other at the lowest frequencies being reproduced, the two subs will effectively act as one larger sub. What this does in real terms is provide a 3db increase in efficiency and a lowering of the frequency where just one of the subs would start to roll off.

The two subs go deeper in frequency before rolloff, and they have twice the dynamic range (output capability). Each doubling of the number of subs will yield another 3db increase in dynamic range and a further lowering of the frequency where the subs start to roll off (this is why I use four 18" JBL subs).

This benefit is not to be sneezed at, whatever the quality level of the subs.

[This message has been edited by soundhound (edited May 21, 2004).]