A guy just posted this on the JVM forum and it seems to make a lot of sense to me:
"Here's the catch - if you have a solid state preamp and a tube poweramp, then the warmth will be added only when the tubes are driven enough to do business. I'm talking serious volume, even with 50W, which means quiet tone=OK, (very)loud tone=better.
With a tube preamp, you will get better distortion at lower volumes (since today's amps rely more on that then powertube breakup) and if it's coupled with an SS poweramp, it will just enhance that sound. Which means, quiet tone=good, loud(er) tone=more or less the same."
It also suggests that having the 100w JVM instead of the 50w should mean that much difference in actual tone. I suspected I'd end up still wanting the JVM but I've been researching some guitar related stuff for a beginners booklet I'm putting together and when I saw Steve Vai and John Petrucci on the Axe FX website it got me wondering how far cheaper modellers have come as it's a couple of years since I last tried one.