I have now worked on two Marshall JCM2000 amps and both are around 10 years old. Both had very similar faults.
The fault on both was that the amps sounded “rubbish” and both were cooking the EL34 output valves. Instead of running at about 45 mA of bias current the lowest adjustment you could achieve was over 100mA. recipe for output valve shortened lives and destruction.
The fault is clearly in the bias circuit. I ensured that the output stage was totally disconnected from the phase splitter stage through isolating the decoupling capacitors. I checked all the components in the bias circuit and I changed the decoupling capacities as they were leaking current due to ageing. However, this only slightly improved the bias current but is was still say to high.
Further investigation showed that the ageing printed circuit boards were starting to slightly conduct and this was knocking the bias badly off. The first cure is to buy a complete replacement board from Marshall but this seemed an extreme move. There had to be an easier way to tackle this. Dr Tube in the Netherlands had found the same issue and has created a little kit that replaces all the bias circuits in Marshall DSL 50, DSL 100, TSL 100 and TSL 122 amps.
There are two versions of the kit , One for the DSL family and the other for the TSL family. The TSL family have a virtual power reduction feature and the board and just by plugging an extra connector then the virtual power reduction remains fully operational
You do need to know how to use a soldering iron and be able to work on an amp. Any amp tech could fit this kit .
I fitted the kit to a DSL 50 and it was a 40 minute job as I had not done it before.
Here is a picture of the board fitted into the main board.