It sounds like there may still be a bad speaker or miss wired volume control. But it maybe in the amp as you say when they adjust it the sound drops out. What is the sound source? They should be adjusting the input to amp not the master on the amp.


Merritt

Business Telephones & Equipment + Commercial Audio/Video Products
Commercial Communications . . . Turner, Maine
If it was built after 1980 don't expect it to work right.