I don't know about LFE channel in theater applications, but for 2ch, everything below 80hz is mono, anyway.
Effects of using y-cable from pre outs
I have built in single channel sub cable. Older receiver only had 1 single sub output. I'm not about to tear into walls or add more cables. If I upgrade to an integrated amp with stereo pre outs...what effect will using a y cable to connect stereo outs have? Phasing effects? High noise?