Marshall jcm dsl 100 head

The amplifier was in storage for a couple of years, following a re-tube by my amp tech.

I plugged in the amp, and it played ok for about half an hour.
It then started making a sound like a bad earth contact.
When I went to switch off the amp, I was surprised with the amount of heat from the proximity of the off/standby swithches.

I took the back off the amp and found a power tube and an effects tube blown.

My amp tech replaced some resistors that had been piggy backed onto three original resistors.

He said that someone had done some work on the head before I even bought it. {I bought it new}

Before I took the head, my tech plugged into a quade box and when he turned on the amp, it immediately blew two power tubes.

My tech said he knew of a guy with the same issue, but he does not know where to go from here.

Can you please assist me with what may be the problem?

Paul Close.