Music Man 212-75 output problem

[hossymandias]

My beloved Musicman 212-75 has been giving me a headache for several months now. It started losing output power so I looked at the service reports and schematics found here: (chassis 2475-75)

http://www.music-man.com/techinfo/old-amps.html

I tried to bias the transistors and noticed radically different numbers on one of the transistors. On closer inspection I realized that someone had replaced one of the original transistors with an unmarked one. I decided to replace with brand new transistors, 2n6488s. When I powered up, volume was restored, and the transistors biased as expected. However, about ten minutes later, volume dropped again even farther. Each channel volume elicits no response until about 4-5, then I get sound, but it is weak. I replaced the tubes, then checked the voltages to the tubes. I read about 250 on pin 4, about 500 on pin 3. This is lower than the schematic, but is it low enough to cause this drop in output?

Any help on the next step? I was thinking signal probe unless you guys had a better idea, as I have never used a signal probe on amps before, only pedals.

[Stevem]

Looks like you have 2 issues with the amp , a SS and a power supply!
If the amp has not been recapped yet then that needs to be done first.
Is the plus and minus 16 volt supply good?