Chris VenHaus recipe power cords made an immediate improvement in my system. His cryoed stuff is supposed to be the shit.
I use a blue circle MR-1200 balanced power transformer and it also made an immediately noticable improvement. If you want to spend less money, there is a smaller wattage version (I probably went overboard) or find a BC music hound to plug into the outlet with your gear. They get good reviews although I never used one, went stright for the music ring instead.
edit: even if you are in an apartment, you might consider installing new outlets too.