Swap out your speaker cable and see what happens - make sure you get a good quality one. I had a similar issue where everything would come on briefly then go dead quiet. I ended up changing all cables at rehearsal except the speaker cab because I didn't have one handy. Almost was going to do a tube roll, then decided to test it on a 1X12 with a different speaker cable - it worked. Brought the same cable to the next rehearsal to try on a 2X12 cab I leave there and lo and behold it did the trick.
Other thing you can try is 60 Watt mode - if that doesn't work, swap out the inner for outer pair of power amp tubes and keep it in 60 watt mode. That will at least test your power tubes before rolling in a preamp tube.
Finally - I had V1 go on me my first few weeks having the amp. Roll one in there, and if that doesn't work, the Phase inverter/driver (V5) is a good one to swap if its on all channels. It's easier to swap tubes by taking of the front cover - something that was a pain with the MarkV.
The other option is to kill reverb and the FX loop to test if it may be something there.
Good luck - I've found my JP-2C to be a little temperamental compared to my Mark V - but both times I had issues it was an easy fix.
