only thing about JCM 800 I'm worried about is getting from cleans like on say "A Toute Le Monde" to the heavy Megadeth distortion. I'd have to stick a pedal infront of the amp. and finding one for under 500 always seems to be a pain for me. Even on ebay.
As far as value for money is consearned i'd definately go for a JCM800, said it before and wont shut up :P
I have the 2205 50w JCM800 which i like better then the 100w ones tonewise, they are often cheaper then their 100w bro's still finding a decent one could take a wile. Allthough you can 'unmod' a modded one be carefull because some DOY modders take out essential parts and toss them while modding instead of a simple bypass, they will be hard to replace.
I always play a stompbox OD with it, just for versatility purposes, cant imagine why anyone would be hesitant to use one. Also try a quality type footswitch, a friend of mine made one for me, but the cheap single footswitches can be noisy.
A JCM combo is an option allthough you should consider a speaker replacement up front, the stock speakers in those are awfull IMHO. With some v30's in it and maybe an extra cabinet you might have even more value for money. Still, if you gig alot a combo is very heavy to drag around, my back still hurts if i only think about that cr@p Peavey combo i had when i started gigging.
The JCM800 to me is a classic, everyone should have one, or two!
