Here’s some highlights from the jea48 linked Tubes Asylum discussion:
-check the bias as they power up, and let them settle in for a bit (like 20 mins) for a final check.
-do not buy new tubes for hi-fi amps from guitar tube suppliers. Buy them from hi-fi tube dealers. Tubes (even matched quartets) sold by most guitar outlets are specially selected to distort early (low transcoductance tubes), therefore they run out of bias adjustment pretty quickly on a HI-Fi amp and there goes another tube!
-Manley has checked out the amps (one of them twice) and says there isn’t anything wrong with them.
-I have one Snapper that has been back to the shop once. It seems to be holding its own ok in terms of not blowing tubes. The other amp is a different story. It just came home from its second trip to the shop and after two hours of use, blew its B+ fuse and two of the tubes, separating the center post of the two tubes from the tubes themselves.
-Try plugging the monoblocks directly into the wall outlet and check the bias to see if you have better bias stability (bypassing any power conditioners).
-When you pull output tubes in and out on a regular basis, we have a tendency to stretch out the tube pins and even can cause tiny stress cracks on circuit board..Now this leaves you with low bias or intermittent bias or no bias voltage many times therefore causing tubes to burn up. I don’t know how much technical knowledge you have on this kind of stuff, but I would re-solder the tube socket connections and tighten the pins inside the sockets or even replace the sockets with better ones
-Manley’s have not-so-good sockets in them.
-yanking tubes in and out of both amps, he could very well have bad sockets in both or loose solder connections at the very least
-We are rebiasing the amps each time we replace the tubes. We recheck the bias every few hours and the amps have never held bias well. We are terrified of damaging the output transformers, power supplies, speakers, and everything else. My husband and I have been audio enthusiasts for almost 30 years and the hobby is becoming more stressful than fun with the problems we have had with these amps. We have spent many thousands of dollars (money we don’t have) replacing tubes by the octet since we bought the amps, not to mention the shipping back to Manley.