OK, loud hum is usually one of two things. DC on the speaker - a "blown output" - or a loss of filter cap on a power supply. COUld be a bad cap, but more often just solder to it.
Until you know the amp is NOT puting out DC< do not connect a speaker. DC will burn out speakers.
So. First thing, test the speaker. Pull the wires off it, or if this is the head version, unplug the speaker. Touch a 9 volt battery to the spepaker terminals. A working speaker will pop or thump with each touch. Don;t leave the battery touching, it will drain the battery and won;t help the speaker any.
Now fire up the amp. Is there DC voltage on the output? I'm not concerned over a few millivolts.
Now check power supplies. It is normal for Vregs and R20/21 to get warm or hot. The schematic says the Vregs are 12v, but they could be 15v ones in your amp. Are both of them putting out their voltage? Look at the various 8-leg ICs. As far as I recall, all of those will have power on pins 4 and 8. SO check a few. Is power distributed throuh the board to them? Those are the low voltage supplies. There are also +/- supplies for the power amp. I don;t know what they are, I'd guess 35-40v or so. Are both present and similar voltage to each other?
There is a tube in this thing. Is the heater inside the tube lighting up? There is a heater fuse if not. And also check pins 1 and 6, is B+ voltage present? I forget what it is, 250-350VDC I'd expect, just don;t want to see zero there. There is a fuse for that high voltage supply as well.
Look at the power amp page. By the input see R1 meets C2, with T16 to ground between them? T16 is a mute, it has to have a voltager on its gate to turn off the mute. Is ther voltage on T16 gate leg? And with the amp powered, measure resistance across that JFET. From the junction of E1 and C2 to ground. Is ther at least a few thousand ohms? Or is it something like 12 ohms, shorting out your signal?
If that is OK, plug a signal into the thing, and connect the FX send to some other amp. SOund come out over there?