Just found a Marshall JTM-60 combo for $645.

It's got 3x10's speakers, and looks pretty old. Thinking about trading my JCM900 head for this.

High as tits
Don't. It sounds fairly fizzy when overdriven. It also is notorious for its overheating problems. I owned one for a week and returned it because the circuit board had been burnt from overheating.
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If you're prepared to spend a few quid fitting an external fan it's alright but I'd personally pass on that deal. Marshalls don't sound too great with ten-inchers either, totally the wrong type of speaker for the tones they typically produce. If you're currently a JCM900 player you may find the cleans a little better, but the overdrive a bit on the raspy, farty side.