Audible low level humm from Vincent st-70

I'm experiencing a low level hum from my speakers when connected to my Will Vincent st-70 that is audible when not playing music. It is constant and does not increase or decrease with volume setting. Replacing the amp with a SS amp eliminates the humm. I have tested all the tubes and replaced the ones that showed weak. All current tubes show strong with two of the Kt77's showing minor grid leakage but still well within the good range. Humm is in both channels. The amp sounds great but I find the humm annoying between discs. Even though my GZ34 rectifier tube tested good could it be the problem?