Power Purpose

I have done the same thing Stringreen. And for anyone that wants to know, I am not going to give out any scientific proof of anything. Only have to please the two things on each side of my melon. They is happy.

I ran new 20 amp, 12 ga lines from the panel to my system.  It did clear up the sound a bit.  YMMV - you might go to all that work and get zero improvement (I did it because the distances weren't long and I was having rewiring done anyway).

Stringreen - 
Not saying there isn't something of a more fundamental nature going on between the two outlets; on the other hand, I wonder if it could be something as mundane as wiping the plug blades via unplugging and plugging back in thereby providing an improved connection?

Could also be the wiring to the duplex. The old one may have loose connections, it may have been old and a lot of internal corrosion. The new one might have cleaner tighter connections internally. The old one may have had a lot of noise on the AC line, and the new one does not. And yes even unplugging then repugging the plug can clean off the tarnish enough to make a difference.

When was the last time you cleaned your contacts? People clean so infrequently that sometimes even just the cleaning action of unplugging and plugging cleans things resulting in better sound.