I tested out a JVM 410 head pretty extensively when I was amp hunting. To be brutally honest, I wasn't that impressed. Don't get me wrong, it's be no means a bad amp, but given the great reveiws it had across the board, I was expecting so much more.
The drive channels sound very similar, the 3 different 'modes' were, to my ears nothing, more than gain stages, and I found it very trebley. The OD channels are certainly what you'd call modern-voiced, and the overdrive itself was very fizzy.
On the upside, the clean channel was quite impressive, and the digital reverb was excellent. Not enough to convince me though, I'm afraid. If you like the modern Marshall sound, you'll probably like it, but it wasn't for me.