I'm going to say that it would take me a while to see how I felt about the JP. I've used Mark amps for a long time and I HATED the Mark V when it came out in 2009. Sold it after a few months and used IIC+. III's, and IV's. Fast forward to earlier this year and I started playing with a band again and we've been doing shows. I was using a Mark IVb for a while but I started to get bothered by some of it's shortcomings live. I saw a cheap Mark V locally and it was mint, $1300. I tried it out and thought, sure, I'll work with it. I got it back in January and I'm still using it and sold my IV. It took me a while to really, I mean really get the hang of it but I think I have a good grasp for the V.
My Rule #1 with the Mark is if I'm not playing with the band and I'm not blasting the shit out of it then it goes in 10 watt mode at half power. At lower volumes 90watt mode and full power just does NOT sound good. I think sooooo many people, like myself, took the V home, set it at 90 watts watts, full power, but kept the volumes to about TV levels, or not to piss off the family level. IMHO the V WANTS to be cranked in 90 watt mode. It sounds stiff, uninspiring in 90 down low. The Mark IV on the other hand I found does indeed sound great at everything in full power but just slowly turning the master volume up. The V KILLS when it's pushing some volume and if the JP2C is anything similar then I bet it's the same case.
Live with a band the V is soooo much better than the IV for several reasons, some may not be a big deal but they are to me. Like it's a LOT easier to plug the foot switch cable into the amp, the handy little hooks to wrap the power cord around in the back, The clean and crunch channels which are a lot better than the IV. Lead mode....yeah I might still the IV a slight edge but I feel with tweaking and using the power modes I could get close enough to any IV after owning soo many of them over the years. And also, do the preamp swap where you put a 12AT7 in V4, I think it helps a little.