Any sub with speaker-cable type of inputs and outputs should be fine.
I agree with John, Eric and Ken.
It takes pretty sophisticated logic to provide not only digital conversion in Zone 1 BUT ALSO independent Zone 2 with digital passthrough if Zone1 source = Zone2 source. It seems like the Emotiva preamp was supposed to do this, but I couldn't find any others (Eric?).
I got around the problem by just connecting relevant devices via both digital and analog cables; the 805 and my old Denon seem smart enough to use the digital source in the primary zone and route the analog to Zone2.
