The Blue Circle PLC (powerline conditioner) Thingee FX2 does a remarkable job of lowering the noise floor for a very reasonable price. And I see they've added a higher level unit on the website. (they're funny looking, but they work).
Their retailer is at
http://www.tlp-audio.com/powerConditioners.php
http://www.bluecircle.com/page151.html
Their retailer is at
http://www.tlp-audio.com/powerConditioners.php
http://www.bluecircle.com/page151.html