Effects of using y-cable from pre outs

I don't know about LFE channel in theater applications, but for 2ch, everything below 80hz is mono, anyway.

As Ghostrider indicates, if the pre outs are part of the main signal path (i.e., if they are jumpered to the inputs of the power amp section of the integrated amp), the result would be a mono signal going to BOTH the main speakers and the sub.

If the pre-outs are separately buffered and are not in the main signal path through the integrated amp, using a y-cable will PROBABLY work ok. But I would first check with the manufacturer of the amp, to make sure that connecting the two outputs together will not cause damage or compromise long-term reliability. It's usually ok to do so, but conceivably might not be depending on the design of the buffer stage.

A mixer, with high input impedance and low output impedance, would be the most ideal solution.

Regards,
-- Al

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