There are many multi-channel PWM amps available. You can get them from LC, Tripath, D2Audio, Zetex and others. With regard to the mains question, this will depend on the amplifier used. Some switch as high as 1MHz, which is no problem to filter out, and additionally won't interfere with AM (unlike some which switch directly in the AM band, often at 384kHz). Switch Mode PSUs vary too. Chord seem to have no problems with them...
Clocking depends also on the amplifier chipset. Jitter becomes an issue, and so yes, you can reclock some PWM amps much as you can CD players.
There are other interesting things with switching amps. Output impedance is high relative to a feedback transistor design, which leads to low damping factor. There are things you can do to mitigate this somewhat, but you won't reach the same sort of damping factors that a top class AB amp can.