Geoff,
I haven't gone to the length of cleaning all the outlets in the house, but I did get a nice improvement with just the outlets on the same breaker circuit as the audio system. Rather than cleaning them though, I replaced the non-system outlets with spec-grade outlets, and the difference was quite noticeable. Not sure if it was due to dirt/oxidation, or due to the crappy contractor-grade outlets with the Romex shoved into the spring clip terminals in the back. I stripped down to fresh Romex and tightly wrapped the wire around the screw terminals of the new outlets as it SHOULD be done. I attributed the improvement to the fresh copper and proper mechanical fastening, but dirt and resultant arcing certainly could have been a factor as well. Took a little time, but didn't cost much and the result was well worth it.