On power up there is a loud humming even with volume at 0

Yes it can.....but you will need the schematics and some test gear in order to do it.....and a soldering iron....what do you mean by no surround on the speaker?? Can you post a pic??

with or with out a guitar plugged in

Why was it on for over a year :D Has anyone replaced the fuse? In the service manual I found it has to be only 2 Amps! Okay let's solve the problem. The power transformer was overloaded which caused the broken winding. It is still overloaded what can be heard by the hum sound. First thought was to check main capacitors or better just replace them! If the hum gets louder, they might also become warm. Check voltages after bridge rectifier and smoothing circuit. They should be +/-42 V (TP3, TP4) and +/- 16 V (TP5, TP6) Let me know which of them is faulty Cheers

Likelly the TDA7293 chip in the power amp has failed. It shorts to one power rail and drags the power down where it hums... this CAN damage the speaker if left on. Verify the problem and replace the chip. Slight chance there are bad electrolytic filter caps but those are easily spotted.

Hum is always connected with the presence of AC interference into the signal path of the amplifier, which means the audio amplification is mixed with the ac ripples.
So this can be a break down with the power supply as a possible tank capacitor or a stabilizing capacitor in the main power or the sub sections of the amplifier or pre- amplifier.

It sounds like a filter capacitor(s) has failed. The electrolytic caps dry out and cease to perform allowing unfiltered power to reach the preamp sections and get amplified.

Being this is tube with high voltages, an experienced person should work on it due to shock danger.

I assume you checked the tubes for shorts as well as emission since a heater to cathode short could cause this as well.

Likely the power adapter has failed if it uses one. Try a new one first.

If half the rectifier blows out, a very impure DC voltage is applied which will be heard in the audio output.

Make sure you have a ground prong on your power cord. Most input jacks nowadays are closed circuit. They have a switch built in that won't allow any sound without the cord plugged in. You may have a damaged jack that will give you a loud hiss, a loud hum or crackling depending on what volume your controls are set at.

try placing your ear on the speakers, adjust the volume control up and down several times, you should hear a humming sound that goes up and down. if not problem with the stage before the amplifier, that is the sound processor. if you dont hear the hum. problem with the amplifier and speakers, including the sound ports or jack. dissassemble the unit. look for loose connections from speakers to amplifier.

Check all the solder points on the capacitors in the power supply. I'm talking about the big can type caps. I've seen this problem on a lot of amps. Good Luck.