When I turn the little dial on "presence" and "brillance" I get a scratching sound from the speakers. Once you are done turning the dial it stops. What do you think is causing this? I have had these speakers for over 30 years and I ain't given them up. No way ! Do you think I should replace anything in my monitors? They sound as sweet as the day I bought them except for this scrathing sound. Still powering them with an Adcom GFA 555 that I bought at the same time I purchased the 100's.