I'm not much of a tech guy, but somewhere in these forums I found this link: http://www.tone-lizard.com/Ultimate_JCM800.htm

This is about modifying the JCM 800 to get more gain (without adding tubes). If I would even consider something like this, I would go to a pro. Does anyone here have experience with mods for the JCM 800? Do you not get the same sound result if you use a good OD or EQ?

well, alot of other amps are modded to sound like JCM800's. If you've already got one, what's the point?
Peavey 5150
Mesa Mark IV
Mesa Single Rectifier (Series 1)
Fender Custom Shop Tonemaster
Roland Microcube

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Digidesign Eleven RackAxe Fx Ultra
Ive seen the "higher" gain JCM800s listed on ebay for higher prices for ages, as far as i can see they dont sell great, i would never buy one, the originals are the best, if it aint broke dont fix it and if you want higher gain look elsewhere
Diezel, Motherfucker
on ebay everyone makes their item out to be the best version of whatever they're selling.
I know Vai used a heavily modded JCM 800 on Passion and Warfare.
Doesn't Voodoo amps do a mod?
Someones knowledge of guitar companies spelling determines what amps you can own. Really smart people can own things like Framus because they sound like they might be spelled with a "y" but they aren't.