Re: Marshall JTM 30
The JTM30s are great little combos for rock and blues. I think they have EL84s in the power stage. I have a JTM60 combo (JTM30's EL34-powered big brother), and it's the most versatile tube amp I've ever owned.
I wouldn't want to compare a JTM30 with a JCM800 or JCM900 - it's smaller, it's different, it was built to a different budget.
The great thing about JTM30s and JTM60s now is that they are discontinued and no-one famous plays them - they're great versatile amps and you can pick them up quite cheaply.