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.
5 responses Add your response
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).