Here's a nice clean example of a 1986 JCM 800 Lead Series, in for a pre-sale service. It hadn't been used for years and now it had poor tone and a fairly loud hum. All the components checked out good but the output looked very poor on the scope. A new set of JJ EL34 and all is well again! love the clean channel with a splash of reverb. The high gain diode clipping channel sounds best with all the treble rolled off to my ears, and is actually pretty good.
I think you'll agree, inside is mint! The only damage was to some of the pots had bent shafts and a couple had broken splines with insulating tape to keep the knobs on. They all worked fine after a spray cleen and their nuts tightened down (painful!).