I was playing my epi boneyard through my jcm 800 the other day, switching between pickups, and i started getting a scratchy, popping type of noise and the volume seemed weak. I've read that the toggle switch on the boneyard is crap so I plugged it into my DC 5 and it sounded great, no problems. The old marshall has GE tubes in it so I have no idea how old they are. It isn't microphonic, I don't know much about tube amps I've had these two for about a year and a half. Would it hurt anything to fire it up and try it again, I really don't want to screw my amp up.
I plugged the boneyard into the mesa/boogie DC5, and had no problems. shoulda said it that way before, huh
If that scratching popping noise only happens when you switch pickups, then i imagine it would be the selector switch. Tube failures usually mean a drop in volume and sound quality. I've not looked at the back of a JCM800, but if you can see all the tubes, look around the back next time it's switched on. If all the tubes are lit up in their normal colour, then it isn't a tube failure.