Sam, the fact that only two amplification channels can be assigned to play speakers in zone 2 doesn't mean that a sub can't be used there. You'd also connect speaker wires from the 507 output terminals that are connected to the zone 2 M22s to the speaker level inputs on the sub, or if the sub also has pass through speaker level outputs, you could connect the M22s using them, rather than directly to the 507. You'd have to adjust the sub's variable internal low pass filter control to try to get a good blend with the natural bass roll-off of the M22s.
