I have one of these new little amps and recently it has started cutting out when I hit the strings on my guitar hard. The only thing I have changed in the amp so far are the tubes. I replaced the 6V6 with a NOS RCA blackplate and the 12AX7 with a JJ. The amp cuts out completely when I am plugged into the high gain input with the volume at 3:00 and I hit the strings hard. Ive also noticed that I can get the sound to come back by tapping the 12AX7 with a pencil. Now Ive tried swapping in other 12AX7 tubes which In know to be good but the symptoms persist. I then put those 12AX7s in another amp I have and they were fine. So at this point I believe I have ruled out tube failure but I'm not sure where to go now. If someone could help me with this I would appreciate it greatly.

Thanks,
Karl

P.S. I have a PDF of the schematic if that would be helpful.

That's a pretty new amp. I'd take it to a tech for warranty work.