While a one-box receiver setup is one way to go, I'm not sure if it would be better or cheaper than a separate subwoofer EQ system like the SMS-1 or that newfangled SVS thing, or even a BFD.

One thing about the Y-adapter scenario is that it will drop the voltage sent to each subwoofer, which may or may not be an issue relative to auto-power-on-sensing.

bibere usque ad hilaritatem