I have a Marshall JVM. It is a fantastic amp. From pristine clean, hard rock 70's style crunch, tight classic metal, liquid butter sustaining solo tone, crushing gain for extreme metal, to over saturated tone for Doom metal. Its all there.
You need to run an ISP Decimator in the loop (or something akin) to kill the pre amp gain noise for the OD channels though. That, along with a NS-2 in front of the amp, and you are ready to go. Run it with either a Vintage 30 loaded cab, or a 1960 cab with the G12T-75's and you are good to go.