Unfortunately they are not so cool because they run so hot and they suffer from it. Look at the picture left. What you are looking at is a power valve that has had its label “fried” off it.
The cause of this problem is that the amps were originally biassed to run a little too hot and also the ventillation is limited with heat rising up but with almost nowhere to escape to.
One of the main problems can be the bridge rectifer shown below. This is black square on the right hand side of the picture. These components supply the pre amp heaters with the super quiet DC. Orignally Marshall fixed these directly to the boards and they get up to around 70deg celsius. The problem with this is that the heat so close to the PCB is enough to start melting the solder joints.
Later Marshall realised this and raised them off the board by a few centimetres, the impact was tremendous and reduced the temperature by about 30%. I go a step further and add a little heatsink to them too.
The symptoms of the overheating is faults like cutting out, channel switching problems and general weirdness. Once modified to remove these issues you have one of the most well thought out practical gigging combos out there.