Sorry - I did not read all the posts...
I used a JCM900 100Watt for years here in the studio - often just to amplify a Valvestate or something like that.
All I can tell is: That amp can sound incredible as well as total fucked up... It depends so much on the tubes and how that amp was attended over the years.
That particular amp was owned by a band-colleaque for a long time and that amp sounded like shit. Then another friend purchased that amp and gave it to a specialist who actually knows his stuff. He changed all the tubes and maintained that amp etc - after that it was a brilliant sounding amp. Because of the absence of a band-room the owner lent me the amp, I had it in the studio for years. During that time I recorded a lot of rock/punk bands which had that amp - either in 100 or in 50Watt. We always ended up recording the stuff with my amp because it always sounded better. And each JCM 900 I checked out sounded completely different.
Over the years the JCM 900 started to go downhill, soundwise and those potentiomenters ets. My friend, the owner, gave it to a specialist again and that guy did a good job to gave a new and good sounding life to that old amp...
So: 300 $ is a very good deal - but it depends on the condition of that amp. You should check it out first and you check out the costs for a good maintaince...
