As far as 6.1 sound is concerned I'm in agreement that consulting Outlaw is probably best, but from what I recall it wouldn't matter as all rear information is mono: DTS-ES is 6.1 discrete and Dolby Digital EX uses a matrixed, mono rear channel (which personally begs the question of why bother with two rear speakers if you're only dealing with a mono signal, but that's not entirely relevant). So whether you plug it in to either it shouldn't make much of a difference.

Update: Ok, I double-checked and I'm slightly wrong. Dolby Pro Logic IIx uses stereo rear channels.