I've found the Shunyata Hydras to be amazing -- very substantial improvement in sound. Hydra 2 used is $250 but only has two outlets, though you can add a double plug or power strip with great results. Hydra 4 is around $500 used, I believe. Shunyata Guardians are about the same as the Hydras. However, you'll also need a powercord with a 20 amp connector. A fine cable is the Shunyata Diamondback -- used is about $100-125 or so.
I can't recommend the Shunyatas enough,
I can't recommend the Shunyatas enough,