Sorry to hear about your issues, extremely frustrating! After reading through both posts I echo the thoughts of sns. Perhaps something has changed in your speakers. This is the only part of the chain that has not been swapped out right? If they are around 17 yrs old it could be a crossover component aging. Can you somehow verify the sibilance is coming out of both speakers? How about opening them up to put eyes on the caps, etc? Maybe your buddy is knowledgeable about this stuff?
The dirty power scenario is interesting also. Are there any new appliances in the house? Any construction or changes recently?