Assuming a stereo system with two subwoofers, I've read here that the multisubbing helps energize the air with low frequencies and isn't so much about separating the L and R signals of a stereo program. Can anyone elaborate on this? Is it reasonable to send the same L+R sum signal to both subs?
Yes, in general we don't take localisation cues from the frequency range in which they operate. Multi-subbing can obviously provide more energy, but, again in general, more power isn't the primary reason why people multi sub. Bass interacts a lot with the room (as the wavelengths are close to usual room dimensions) and you want to even out that interaction.
That's really in a nutshell, there is a lot of literature on the issue.
Google "multi sub placement guide pdf" and you'll have a ton of stuff to read.
