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Just got a JCM 900 MKIII 2100 (100w/50w) for a STEAL with a badass peavy 4x12... The owner was moving and the pots are very scratchy so he just wanted a quick sell. Loud as fuck through a 4x12... Sounds incredible with preamp maxed and the onboard gain stage at 0.

The SLX version seems to have more of a following, but the impression I get is that this earlier Mk III version is the best of the JCM-900s. Deoxit is magic stuff for pots, but be careful not to be grabbing at the guts when you take the chassis out to give them a sip--the filter caps can store enough juice to be quite harmful to a person.

I took some contact cleaner to the pots, helped in most case but the second volume knob seems pretty fucked, it jumps to full volume if you press it hard. May need to replace or remove it and take apart the pot. for now I'll just leave it at a setting I like.

The two master volume knobs is a great feature--stomp on a pedal to switch between the two.

I could be wrong but I think the pots on those are the same as on the 800 era amps--they will flake out, but are readily available for replacement. That volume pot behavior sounds just like what I encountered in the gain pot of my Lead 12 solid state combo--Deoxit got every one of the pots happy but that one.

The switchable volumes seems very useful, but from what I've read it seems like the solid state circuitry involved is part of why the 900s aren't as loud as earlier amps. If you need it louder and can lose that functionality, maybe it could be bypassed by a tech or DIY. But I dunno, a quieter Marshall sounds handy to me.