Effects of using y-cable from pre outs

Investigate an Outlaw ICBM - could be your answer.

Won't a Y-cable used in this manner effectively short the preamp outputs together?

Yes it will, Bob, but as I indicated that will USUALLY be ok (assuming the signals being shorted are separately buffered from the signal path to the main speakers). In fact, the literature on REL subwoofers specifically suggests doing that in cases where a speaker-level connection is not possible.

The reason it usually is ok is that the output impedance of the two outputs is typically high enough to limit the current flow to an amount that the output transistors or integrated circuits can handle safely. As contrasted to the situation, say, where the outputs of two solid state power amps (with their extremely low output impedances) are shorted together, in which case pyrotechnics would be the likely result!

Regards,
-- Al

The amps I've seen have a "pre-out" in case you wanted to add another external amp. I'm not sure about "jumping". I believe it's a regular "out" that you can control volume with remote and use internal amp at same time.