I love my 5153, but I've had a bonus and really fancy a JCM900 for extra tonal options (particularly with my band hoping to record in the next couple of months). I'm thinking what I'm after is the SL-X 2100, which is the 100w 5881 powered version with the extra gain stage, is that right?
Basically, I want a fat Marshall scream in the vain of some of the earlier Zakk Wylde stuff, but also able to cover 80's shred, grungier Alice In Chains type stuff and blend with my 5153 to give me a Carcass-esque tone too. Though admittedly I can get pretty close to a "Heartwork" tone out of the 5153 on it's own.
Oh, I should mention I've got a budget of £450-£500, so a JVM is a bit out of reach at the moment.
The JVM205H is friggin' awesome; definitely worth saving a bit more for. I never really liked the SLX stuff, personally. Too buzzy. Though granted, it's been a decade since I've played one.