Ugh. I have the same amp. I love it! I just hate the shipping and maintenance costs. Could be a capacitor or something. Speak to ARC. You may need to have it sent in for service and have some minor repairs done. I've done this a couple of times with mine...
Using contact cleaner in tube sockets
My ARC VT100mkii which I just got, has a crackling from one of the channels. The tubes are new. When I first installed the tubes, it was fine. The following morning, I powered it on, and the crackling started. I swapped tubes with the other side and it was fine for a while. This evening, it started again. Will using contact cleaner damage the sockets? It is quick drying cleaner.