Two sources, one sub - can I use Y-cable

I have a Musical Fidelity A5 integrated and use the PreOut to connect to my subwoofer. When used in the "Home Theater Bypass" mode the PreOut is inactive (no signal). If I hook the single end of a Y-adaptor into the sub input and hook 1 of the 2 other ends to the MF A5 PreOut and the other to my Rotel 5.1 receiver's sub out am I asking for trouble? (this would provide one signal or the other to the sub). Would the Sub out from the active amp end up "driving" the output of the inactive amp? [I do have a switch box that could used to manually switch between inputs but that does require human intervention!]