I dont' think you can fulfil all your checklist for £600 and still buy new.
THe Rocker 30 is a fine amp, but that is based on the gain channel. The clean channel is a big compromise and is generally agreed to be poor.
The Dual terror I don't know too much about - it'll do the gain, I'm sure, whether it's got a good clean channel would probably be the deciding factor.
if you like the Lionheart's distortion, and it has enough volume for you without breaking up on the clean channel, then it could be the amp for you. Personally, I find the distortion too "loose" and vintagey (in a real 60's way) for me.
JVM and VM --ughh. The Vm is a single channel amp anyway.
Other (slightly more expensive) options for you are a Bogner Alchemist (highly recommended),
or ............ a nice clean amp, like a Hot Rod Deluxe/Blues Deluxe (or an AC30CC) plus a drive pedal or two. It's much easier to get a new drive pedal for a new flavour of drive, than buy a whole new amp. A pedal rig will generally also sound the same at all volumes.