I'm not surprised that trying different power options didn't make any difference in the hiss. I've experienced that having good power generally just keeps static, buzzes and such crap out of the audio signal. It _is_ kind of strange that only your back channel has more hiss, however. Is it just as hissy if you have it set to LB/RB in the speaker setup menu? When I reduced the channel trim gain to -15db, my unit is extremely quiet, but I'd have to make up the gain elsewhere, and I'd be back where I started, hiss wise.