Tube Question

Hum is most likely coming from a Transformer not a tube..

+1.  Almost certainly transformer.  Do you hear the hum when you're close to the unit, through the speakers, or both?  Any which way, transformer.  Could be DC on the line, could be ground loop, could be a transformer that hums.  (Other manufacturers when asked have replied: yes, they do that.)

First, please clarify whether you are talking about a mechanical hum coming from the unit itself or a hum that is audible on your speakers.  If it's a mechanical hum, I agree with the above two responders.  You could try tightening down on screws that hold the transformer laminations together, if the transformer is not potted. Or you could isolate the transformer from the chassis on rubber grommets.  Neither is guaranteed to work but might work.

If it's a hum on the audio signal, then the issue is mostly likely due to equipment grounding, not a tube, although it's also possible you have a dirty tube socket.  All that said, if you've never changed the tubes since the 90s, it's probably a good idea to do that too.

 I would definitely replace those tubes even if it is not the source of your hum. You will greatly improve your enjoyment going with different brands. Both of those brands you mention make very poor sounding tubes in my humble opinion.  Call Andy at Vintage Tubes services as he will be able to recommend the brands for the sound quality traits you desire. 

Thank you for all your responses. Dealing with the transformers is beyond my pay grade. Does anyone have a suggestion on where I should send the amp for professional repair?  I live in the northeast US  but would be willing to ship it anywhere.The company that made the amp no longer exists so that isn't an option.