Yep, loud hum and cone at extreme is classic DC on output. Your amp is "blown". Two reasons mostly. One is a shorted output transistor, easy to check with an ohm meter. the other is good outputs but some part earlier in the power amp is telling the outputs to turn on fully and stay. Another possibility is one blown power supply fuse - "rail" fuse. If one side blows its fuse, then all the circuits snap over to the other side voltage.
This is more complex than many amps inthat it has two sets of power supply rails for the power amp. 40v and 80v. It mostly runs on the 40v, but the 80v ones switch in for peaks. Class G or class H, I forget which. SO with 80v on the speaker, the high voltage is turned on, but as I read the schematic, putting 40v on the output would by itself turn on the 80v. There are MOSFETs for each polarity which turn the 80v on and off. I don't know whehter we have a blown MOSFET or not, but can't rule it out. We check them too.
Fuses F1, F2 are 6A slow blow and need to be checked. Pull them from the clips and use an ohm meter to test them. Don't test them while clipped in.
here is the service manual.