I'm a new Mark V 90 owner. It is the single most impressive amp I've ever touched.
I have previously said that in general I don't care for amps that have more than 12 knobs or 2 channels. But Mesa has changed my mind about that with their logical way of laying out three sets of controls, one for each channel. Gone are the shared control functions that always made setting up the tones of any earlier Mark series amp a balancing act between the R1, R2, and lead tones.
I've previously owned two triple rectos and a 3 channel dual recto. They got me used to this setup. But the rectos were really not quite my type. I got them cheaply enough that they were well worth the experiment. (Sold them for modest profits, eventually, too.)
But to me, the Marks sound better than the Rectos. Tonally speaking, my Mark IV was better than the Rectos, so was my Mark III Blue Stripe. Now that I've got the Mark V, I have the best of all worlds. It'll keep up with and destroy all competitors, whether you play in standard tuning or drop tune down into the subsonic realm. Yes it chugs if you are into that.
I can barely imagine what a hypothetical Mark VI will be like. There's so little I can think of to improve on the Mark V.
It could be that the Mark V is the amp that gives me a reason to sell off all the rest of my amps...except a few pieces I want in my collection forever.
Is the JP2C even really better than the Mark V? The second EQ doesn't seem like a feature I'd really NEED. I use the one EQ, and the EQ presets serve well enough any time I may want to apply a different EQ setting to another channel.