With an overdrive pedal to boost it and a thd hotplate I can get quite high gain tones from the jcm800 at bedroom level.
or even without the hotplate. the 2203/4 takes boosters very well, as is its famous reputation. i have a selection which i choose depending on the guitar i'm using. my hilbourne treble booster is great for vintage voiced pickups, i take an RC booster for vintage hot pickups, and a crunch box with high gain pickups. you can go high gain with the right pedal and still keep the amp's character - early metallica levels easily, maybe more. i like boosting it with a boss EQ pedal too, though thats fallen out of favour a bit lately. but i might well use that eq again soon. i was looking at pics of lynch's rig - and his ge7 turned out to be set to the exact same settings as mine, obviously its something that works and i'm not crazy ;)
master volume/slx model jcm900s are meant to be better than the dual reverb ones, perhaps i'd look for one of those. or a jmp type master volume (like I own now) - real sleeper amps, or a dsl as you say. i also liked jonothan's split reverb jcm800 when he had it, though they are starting to appreciate. likewise silver jubilees aren't the bargains they once were. i agree with everyone that those vintage moderns are awesome, but they're still a bit expensive being new amps and all.
pentode/triode split feature is a total waste of time. it does lower the volume appreciable, but it sucks all the balls out the amp. i'd rather leave it on pentode and turn down the master volume a bit.