It could be the pot. But it's more likely the joint where it meets the circuit board. Specifically the ground pin. You could try to re flow the solder joints on the pins. If that fixes it, your done.
It's probably a standard 16mm pot (I don't know the value either). You can get them anywhere. If you just replace it you can deal with both possibilities at once. Just take it out and measure the resistance between the two outer pins. Then order a pot of that value. If the two outer pins read infinity ohms then it's certainly the pot. In that case, to get the value you would have to measure from one of the outer pins to the center pin. Rotate the pot to find the highest resistance. Round up to the nearest standard value. There may be a part # on it and if you can order by part # then your sure to get the same taper too. Peavey has always been very helpful with me. If you call them they could probably give you info to order the part so you can continue to use the amp until the new part arrives.