I would look for a blown EL34 tube screen resistor. Should be easy to see.
http://www.kbapps.com/audio/tubedataspecifications/images/el34_05.gif
Follow pin 1 on each right channel EL34 tube for the resistor for each tube.
Cause?
Bias is way off.
Tube shorted.
Possibly a bad driver tube.
How many hours do you have on the power tubes? Average life, I believe, is about 2000 hours.