Air tight atm3

The bias level on all of the tubes on one of my Air tight
ATM- 3 mono block power amps dropped drastically and a wiizzz sound is noticeable on the amp and loudspeaker. What could be the problem . The system has less than 20 hours of operation.
Thanks for your comments/ suggestions.
i own a pair of these, the easiest thing to check first would be the driver tubes. do you have a tube tester?

as you may already know, Tom Tutay is the expert and US repair person. If it is not the driver tubes, I would call Tom.