I think I'll conduct an experiment this evening and report back. I have a third sub in the back corner of the room that's used exclusively for home theater. All I would need is a long run of RCA cable to hook it up to my 2-channel system. It might be interesting. I'll play around with the phase adjustment.
OK, I just performed a quick experiment and here's my assessment:
Gut wrenching, bowel busting, ball tingling bass!
I started with some very bass heavy George Duke tracks. The third sub took the bass to a completely different level. It's crazy, man. I like. A lot.
Then I slowed it down and put on the latest Lizz Wright CD where the first track has some great bass. Incredible! I set the phase level where I liked it. And I really like how I can easily dial in just how much or how little bass I want.
Bottom line -- a free tweak that's gonna remain part of my two-channel system.
FWIW -- I'm using a bastardized AV123 sub (remember those guys?). Well, actually, I'm only using the gorgeous cabinet. I upgraded the amp and the 15" woofer a few years ago. I believe Danny has a thread on here somewhere where he re-built the cabinet I have, but I won't go down that road.