Voltage Level at speaker terminals

You are measuring AT the speaker terminals? Across the terminals for voltage and in series for current? What other troubleshooting did you try, or did you go right to the DVM?

Is the loudness imbalance with all sources? or just phono?

I'd use CD and test tones....but that's just me. I'd also measure more than a single frequency, trying well above and below the speaker crossover frequency. Than swap speakers L/R leaving everything else alone....repeat measures. That will isolate to a 'way' of the speaker or L or R side of the amp....

Possible causes? Tube or bias problem in the amp. Or even differences in the 2 pots for vol control. Phono cart imbalance is not out of the question, either.

Interesting thread, but how can you be measuring current if the cartridge is not on the record?

If your preamp is set for CD input do you get the same results? How about when the preamp is off and the amp only is on? Also, you say that you see higher amperage at the speakers. Just to clarify, you have put your multimeter in series with the speaker wiring to measure current? How much difference in current do you see? 2X, 1.5X or some more arbitrary value?