SUPER late to the party, but I have joined the club of JVM appreciators!
I think this amp does not necessarily need a boost, and with clean boosts I get a super weird "whompy" effect, but it loves the Fortin Tempest it seems, which is a rare sight, as that pedal sometimes is hard to pair with some amps. The JS version, which I have as a long time loaner here at my place (and which I will shoot out against the regular on the channel sometime in the context of thrash stuff and a bit left and right) takes pedals WAY better, but on itself to me lacks the punch and "metal-ness" on the OD1 channel of the regular. OD2 does not jive for me on the JS as well, while it's gnarly and mean on the regular.
BUT - the Crunch channel on the JS makes me kneel before the hard rock crunch god tone that it is, while on the regular one I don't like it at all, unless boosted with an SD1.
To me, and that might be controversial, the clean of the regular one is better. Less sparkle (I call it spikey), fuller for arpeggiated chordy stuff like I tend to play. Clean Red on the JS however is a really nice rock tone.
I am super eager to get top learn more about this thing, I am starting to dig into reading tube amp schematics, and using the JCM 2204 and JVM as references to kind of get a grasp on what's going on under the hood. I want to soon bias the amp way hotter, to around 70%, since in my experience I tend to like hotter biased amps. I'll try to document the process and make "cold vs. hot bias" clips and smack out a video on the subject.