ARC REF 150 Buzzing noise

I have an ARC REF 150 amp with over 4000 hours on it, and recently it has developed a buzzing noise.  This appears after about 30 minutes of use, and seems to get steadily louder.  I thought it might be a tube.   I had KT150 power tubes in it and I swapped those out  for KT120's and re-biased.  No change.  I then swapped out the 6H30 input tubes and re-biased again.  Buzzing is still there.  It is very difficult to locate the sound on the boards.    I have also had several blown fuses, this has been in the midst of a move and may or may not be reIated to the buzzing.  I have to assume this needs a repair, but I am wondering if anyone has a guess or even better, experience with the problem.  I am in the Denver area and if anyone knows of a local repair opportunity, I would appreciate knowing that too.  I do not look forward to the expense and delay of sending it back to ARC repair.  Thanks.  Scott
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Update on the sad story of my Reference 150.  Took it to Audio Tube Supplies in Denver, they determined that one of the input tubes was out of spec, causing one of the KT150 tubes to become over-biased and over heat.  This in turn (educated guess) caused one of the rubber washers under the transformer to shrink, and the transformer began to hum -- after it gets warm.  In summary, the bad input tube caused the amp to get too hot. The shop placed a sliver of heat resistant neoprene under the washer, and the problem went away.

Lesson learned, check your tube bias regularly and often!  Especially if your tubes have some hours on them. 

It now has a new set of tubes.  I felt cautious and went back to KT120's, although the shop verified that there should not be a problem with using the larger KT150 tubes.  I don't think there is any permanent harm, the amp sounds as good as ever with the new tube set.  But, I am going to send back to ARC for complete check.  It has over 5000 hours.

@scottwsmith I'm having the very same problem right now. Buzz in about 30 minutes, sound gets leaner as the buzz is getting stronger. 

Good to know where to focus. 

KT150's should be alright, I checked with ARC some time ago but I'm still using KT120's as they still have lots of time left. 

KT150 are just fine to use in the Ref150. In fact they should sound much better than the 120s.