I have not played a IIC+ so I can't make any judgements on those and how these amps compare to those.
I agree that I found the JP to be boxy. Then when I got the VII and played them next to each other, it wasn't even close. Which was a bummer because the features on the JP are nearly perfect. I really wish they would've kept the 2 EQs and Shred mode for the VII. The VII is absolutely amazing, and if you want the history of Mark series amps in one box with a shit ton of modern features, this is THE way to go. What an overwhelmingly amazing amp.
The IVs are amazing amps. Probably still my favorites, but it's been over 6 years since I've had those amps. The V25 is a great little saturated monster. The 35, too. Very similar but maybe a little more of that EL84 squish.
I have limited experience with the III, and I thought it was cool but didn't really offer much that the others offer in a better way.
Currently I own a V. It's my second one. I hated the first one. So harsh and loose while not having a very big or punchy low end. It got dropped in shipping, and I think something must've been wrong with it because my current V sounds nothing like that one did. This amp sounds great, and has Extreme mode, which is unique from all other Marks. The IV mode gets close enough to what I remember from my IVs when I want that sound, but mostly I've been living on the Extreme mode and cranking the gain almost to the max haha. Its been sounding and feeling great, and both bands have been saying they prefer it over my typical EVH 100S. IDK if I feel the same way about it, maybe they're closer to equal for me. Both great amps.
Anyway, the IV is still probably my favorite Mark, but I would need to compare it directly. The VII is absolutely amazing. If you just want classic Mark sounds in a modern package, that is 100% the way to go. It gives you everything and sounds great doing it. Plus the built in direct features are extremely useful on stage and in the studio. Extreme on the V is not what I would consider a classic Mark sound, but it's unique and sounds great, so I'll be sticking with this one for a while.