Quote from: g1 on October 24, 2023, 06:29:25 PMIs B-C junction of Q9 giving proper reading on diode check?  It seems like Q9 might be open.

You nailed it!
I hadn't checked Q9 out of circuit. In circuit I was getting around 300mV in both directions which I suppose is not ideal. I guess an open circuit is less obvious than a short. I desoldered Q9 and the Collector leg broke in half. It wasn't making contact - hence the open circuit! But luckily there's enough of a stump on the transistor case to solder a wire to, so I tested the transistor and it's fine. Just soldered it back into the board and all the negative voltages in all the wrong places have vanished. Gotta put the amp back together and see how it sounds now!
Thanks for all the help, g1!
I learned a lot here. I didn't expect the open circuit situation. I haven't seen that much of it before. I guess a clue was that there was negative voltage all over the place but it wasn't drawing much current. Nothing was getting really hot. A short circuit would probably be clearly different in that way. 
 

Yes, a short would have probably created some heat somewhere and maybe even fuse blowing.
Glad to hear you got it working.