You could probably do that as long as you can remember to only have one pre at a time turned on. A better solution, would be to use a sub equipped with both kinds of inputs: one set for L/R 2 channel line inputs (from your MF) and another LFE (low freq. extension) single input for the sub-out from the Rotel. These subs have internal buffers to keep the two inputs apart AND (just as important) separate contour controls for each. There are several units made this way. The Depth or Descent from MartinLogan are one example.
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!]