Is two Subs better than One?

Lynhnn, just curious, but at what frequency to you cross over your subs to your main speakers?

How many "general concenses" would you like? There are definite criteria for either choice ;-)

If your mains peter out at 100Hz or higher (or need to be high-passed above 100Hz to avoid distortion) then you definitely need two subs to preserve stereo directionality (which is present down to around 80 - 100 Hz) The cons are that two subs are hard to place so they integrate seamlessly with the mains/room -- the biggest issue being phase matching to the mains at any given location.

If the mains go low enough to require sub-assist only below 80 -100 (or preferably 50 - 65 Hz) then one sub is preferred. It can be placed almost anywhere, but right between the mains is preferred. If you can't do that, then place the sub at your listening position, and walk around the room noting all the spots where the bass sounds best. Use one of those spots to locate the sub ;-)

Foster, OK then my answer is "not necessarily". Which way is actually "best" I think depends on the sub's selected low frequency cut-off point.

For filling in low frequencies up to 50Hz, one sub is better. Since the brain can't localize a source below about 80Hz, placement isn't that critical as far as imaging is concerned.

Above 100Hz, two are definitely better than one. Between 50 - 100 Hz it's a tough call, but I'd still start with one. Then, if there's insufficient low-frequency imaging (meaning the bass doesn't seem to come from the mains) then two subs placed somewhere near each main will provide more realism -- but then room acoustics will require more adjustment.
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Baam, one v. two depends on the high-freq at which one plans to roll off the sub(s). Once that decision has been made, the size of the unit(s) regardless of whether using one or two, really depends on what SPL (sound pressure level, in Decibels) one needs to reach in a given setup/room. And THAT decision will be determined by the type of music on listens to, and whether HT will be a part of the system or not.