In general, the longer a speaker wire run is from amplifier to speaker, the thicker the wire guage shoud be. Does the same hold true if the wire run is from a preamp to a self amplified speaker?

Recently moved my front L/R self amplified speakers to the back of the room. Run is now about 40 feet. Technically, I'm not using speaker wire, but using rather thin speaker cable with RCA phono plugs running from the speaker to the preamp output of Yamaha 2500 a/v receiver. Would I benefit from thicker cable? (thicker/better cable might get into substantial $$$ when you're talking 40 feet x 2 runs).

Also while I'm here. I'm getting an audible hum from the self amped speakers after I power down the receiver. One speaker won't go into automatic standby because the hum is too loud. The other goes into standby, but the hum is still there. When I turn the receiver on, the hum goes away.

The self amped speakers have an on/off switch and I could manually turn them on and off every time I wanted to listen to music, watch movie, etc. Would rather just leave them in standby like I did when I had them up front. Only difference from before is that the cable run is much longer (with one coupling on each side) and the self amped speakers are plugged into different wall sockets than they were before.

Any suggestions?