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It's fixed!!
After thinking about it for a while, I decided I really ought to replace those original 6BQ5 audio output tubes. I had read that the 6BQ5's are commonly used in guitar amps, so I made a phone call to a local guitar shop. They told me they had 4 of them in stock, for $7 each. I ran on over there to get them ($7 a piece is the best price I've seen on them). Three of the tubes are new production "Mesa" brand tubes and one is a NOS General Electric tube.
I knew when I powered it up the first time I made a wise decision. The volume control (which was set to almost the center position before) had to be cranked down. The sound is twice as loud and twice as clear as it was before. I had to turn the bass control down quite a bit to keep it at a comfortable level (it has been set wide open with the original tubes). With the set of 4 new output tubes, I had no problem at all adjusting the current balance control to exactly zero volts.
I flipped the chassis over, and lo and behold....the B+ was down 12 volts from what it had been before. This is much more in line with what I felt was a comfortable increase considering today's higher line voltages. I also noticed that after being turned on for an hour, the power transformer was still relatively cool to the touch (it was running pretty warm with the old tubes).
I am thrilled with how well this old girl is performing now!
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