Passive line conditioners help a little, can eliminate some high-frequency noise, protect against spikes, things like that. If you really want clean power, you have to go to something like the PS Audio "P" series, which actually generates a brand new sine wave from scratch. And for any of you crazies out there, the ultimate is what's called a "motor-generator", which is exactly what it sounds like, a big electric motor hooked straight to a big generator. It not only will deliver a pure sine wave of tremendous current capacity with very high efficiency and reliability, but also provides 100% absolute line-to-load electrical isolation. I'm surprised nobody is selling these for audio use yet. Obviously, it can't be placed in the listening room, but that isn't a problem that can't be solved.