Bad Hum/Buzz on Joule Electra VZN 100??

guys just got a pair of VZN100 mk4 or 5( green board)-one of the amps buzz loud enough to hear while music is playing-I get the tubes to bias but when I check the DC offset it either reads 1 or bounces all over the place- when I push one of the bias buttons down the hum lessens but it is still there-any ideas???- I changed all the output tubes-
I would call Signature Sound, they do all of the service now since Judd Barber retired. Maybe they would give some ideas what to check and if needed they can repair the unit for you.

Very nice amps by the way!

Good Luck, Greg