I did get to try it out with with and without a boost out front, and was able to play at band volume. Sounded very thin, even with the bass rolled up. With the bass all the way up, it was a sort of "warm-thin" tone. The sweep of the bass adjustment was pronounced, not so much the mid, while the treble knob seemed to do nothing. Not enough gain for me either. Turned up the gain on the boost, and the rig sounded basically like a cheap distortion pedal. I also got a ton of feedback. The screechy kind, not the good kind.
The amp had seen better days, so I'm thinking there might have been something wrong with it. At any rate, it wasn't the amp I was looking for.
So far I've played thru a couple of JCM 900s and this one 800. I didn't get along with any of them. But I do love that Marshall sound, ironically. I love it. Just when someone else is playing.