It appears there are proponents for connecting two Subwoofers both in stereo and mono, so I will most likely experiment myself.
I initially asked the question because I was thinking in terms of a vintage system I have, which has no Sub outs on the Pre Amp, and only room for one set of speaker connects on the Power Amps. Thererfore, I'm looking for Subs that have Speaker level Inputs and outputs.
A side note: Its been frustrating researching Subs for this music only system. Everyone always talks in terms of "This Sub will bury that Sub". I don't want a Sub that will bury anything. I just want them to supplement a vintage Two Amp, four Speaker music system that lacks low end.
There seems to be allot of love for REL. I had decided on SVS to supplement a different music system I have that features Marin Logan Motion 60s. I will have to look into the REL's. Thanks.