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!]
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.
No. The output impedance of the pre-out may quite low and will drop the active signal level going to the sub.

Y's in audio are a nightmare. You can only use them to split signals from one source to two high impedance inputs.

If you want to connect outputs together then you need a resistor network box to maintain impedance levels remain high enough so that no source gets damaged/distorted.

See this Passive Mixer

or, if you want to do it properly then you really need a high quality active analog mixer (these are used in studios everywhere for patching gear).
I agree with Shadorne. Don't do it. You can buy a simple switch box if need be.
I use an NHT SubOne(i) which has both LFE and two channel inputs that can be connected at the same time. It is a discontinued model now, but I'm guessing the current NHT subs offer the same feature.
I have multiple source material (ie HT and my main 2-ch listening system. I use aSescom stereo selector box to manage hte I/O. Works great!