Help me diagnose the problem

I am going to move the suspect KT90 to a different tube socket. If I continue to blow the V1 fuse it's not the tube. If I blow the fuse corresponding to wherebI move the tube, It's the tube-

could be a loose solder joint on the socket - that happened to me with a tube preamp

It sounds like the tube is shorting and blowing the fuse. It may be time for new tubes.

I'll state the obvious, have you called Rogue back?

I agree with the comments above. My amp does not have separate fuses for each tube. So, if I blow tube because it arcs, there's a good chance I took a bias resister along for the ride too. And I agree that changing out a bias resister is an easy fix.

As an aside, I've had a brand new tube arc and blow a bias resister. But more often it's old power tubes that are more likely to arc.

ARC recommends that I change out power tubes every 2000 hours. Don't know what Rogue recommends, but you mention in the OP that your amp is 4 years old. Have you changed out your power tubes yet. If not, you may be way, way overdue.

Otherwise, the advice above is reasonable, including calling Rogue if you are not able to easily resolve the problem. FWIW, tube amps are pretty robust. The only repair issue I ever had with my amp was a blown bias resister -- and that can happen anytime.