Monty, I purchased up a Monster HTS 3500, primarily for surge protection, on close-out a couple years back. I can't say that it did anything to improve the sound, outside the hum isolation. It worth about the $149 or so I paid for it.

The APC H-15 unit that praedet and XenonMan mention looks like a bit of a step up from the Monster HTS 3500. I'd likely order it if I was in the market again.

Rob