possibly consider a dummy load.
an 8 16 ohm resistor of huge wattage 10000 watts
and rig that in with another resistor to the headphone jack
the rig would be external but it is a way to jack sweet tones and not at that loud volume
and yeah when it's cranked you hear music coming right from the resistor.

the first thing you need to do it make sure the pot works, the dial/knob that controls the volume.
you will need a soldering iron and a multi-meter, take the amp apart to the board,
you can test the pot inside the circuit and it should read accurately
set the multi-meter for kOhms, and test from one of the outside pins to the other outside pins.
then test from one outside and the middle one and adjust the dial, pots (we call them now we are engineers) and check to see if the value changes from 0-the number you read across the outside legs.
if the numbers don't match or change when you turn it you have a bad pot. replace it,
YOUR AMP SHOULD BE UNPLUGGED FOR A DAY BEFORE YOU OPEN IT
even solid state, there is enough trouble in there to give you a good one.