Have you swapped the tubes for known good ones? Have you tried replaceing filter caps? Any chance of a parasidic oscillation? Connect DVM to speaker and set to AC volts. Reading should be low millivolts with no signal.
Any other possibilities you feel you have eliminated? Do you have an identical amp that you can jumper signals back and forth? Try feeding the signal from a known good amps' preamp to the poweramp and vice versa.