As I've posted before,

some measurements of a power surge protector/AC noise filter:

http://hem.bredband.net/b113928/Surgefilter.htm

It reduces AC transients, but introduce noise. The noise CREATED by the AC filter was also confirmed in a completely different setup. Computer was not connected to the AC mains during measurements (battery). One user reported that the specific AC filter also caused his amplifier to go frequently in protection mode, and occassionaly break. This phenomenon was also noted by me with one power amp. By removing the filter, the amps worked normally again. The manufacturer did not find anything wrong with the filter. Some users have their amps (and speaker) gone up in smoke, but it could not be verified whether it was the filter that caused it.

Conclusion: AC noise filters may not do what they suppose to do, despite the claims.