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-