I'll admit, I bought my V4BH oh a whim and was pretty upset to get nailed with a $300 repair bill, but I know I take care of my amps and know that these tubes will have a long life ahead of them. My tech looked the amp over and fixed a lot of stuff I didn't even know about to eliminate the risk of bigger issues coming up down the line.
My Mesa hasn't seen a tech since I bought it but only had one owner. The fan doesnt run on high while running an 8 ohm cab (a little odd) and I know that when the time comes I'll probably have to drop about $200 to fix whatever the issue is + new tubes maybe. I also know this amp isn't going anywhere until my back cant lift it anymore
I think a lot of people that have issues with their tube amps either bought an amp that was damaged that they didn't know about or it's getting roughed around during load in/load out. I can't see why one of these amps will have any more issues then a regular SS amp would, but when buying a used tube amp you should always consider in the cost of a tech visit, just in case. Heck, even bring the amp to the tech and split the bill with the seller if possible, then you know 100% your amp is ready to go.