I, too, am an apartment dweller with a fairly serious stereo (hey, some things come first!) and found that replacing the outlets made a good difference. I used Hubbell hospital grade long before all the specialist outlets came out. Replace the circuit breaker if it is fairly elderly. Another positive was the Audio Prism Quiet Lines along with a Noise Sniffer borrowed from the Cable Company. There was measurable noise on duplex outlets I wasn't even using! I could only blame "the neighbors". Also check out this site for tweaks: http://www.magnan.com/column.shtml
(He's not just trying to sell cables, either.)
Good Luck! It is definitely worth working on!
(He's not just trying to sell cables, either.)
Good Luck! It is definitely worth working on!