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.