I play in a live rock band with a drummer using a Roland VDrums drum kit which requires a solid bass response out front for the kick drum so we use a 2 JBL 15 inch subwoofers. One may be fine but we do use 2 of them. But if we didn't have an electronic drumkit I think one would be fine.

We do have the sub woofers separated on each side of the stage but that is more because they hold the poles for the 15" PA cabinets. I believe the correct technical answer is ( and you can verify this with a google search ) is that the position of sub woofers simply doesn't matter. Sub bass frequencies are NOT directional like higher frequencies and placement doesn't affect stereo positioning. Also related to this if you are using 2 sub woofers they provide better bass response when placed together rather than separated

Nigel,

Right on. We have been using 2 subs and depending on the size of the hall we place them together (large) or left and right (small). The problem with placing them separate (left and right) especially in a larger room - is if you walk the room while the band plays, you will notice (using your case) that although bass has no direction, there will be areas of 'cancellation' - where you won't feel the 'kick' as much as others.

Now that we have started doing larger venues, I think the band-pass subs have reached their limits.
I have heard the Yorkville LS801p twice now (both times, just a single unit) and I must tell you, you felt the bass on your pants and on your shirt! Of all the powered 18" subs, it appears to have the largest ports too - which could explain how/why it throws the bass that far out. I have also heard the QSC HPR 181i, however these were a pair and the bass (although not as punchy) was very clean! Now, ideally you would want 1 Yorkville and 1 QSC - but it's not advisable to use 2 different subs with the same signal.

Then finally, there is the decision of going with a 15" or 18" horn. For tight and clean bass reproduction (where you can distinguish notes from the bass player), a 15" sub is recommended (faster response than an 18") but an 18" moves more air and consequently will cover more grounds - maybe altimatly (since we have 15" in the main PA speakers already - go with one 18" versus 2 15"...hmmmm ... decisions, decisions ...