They're crap because they're expected to be cheap. It must cost almost as much to make one as to make an octal power tube, but they sell for about half the price. Even the prices of modern preamp and octal tubes are ridiculously cheap, so it's no wonder they're all junk. Compare the prices to those of NOS tubes when they were just 'new', and adjust for inflation, and you'll see what I mean. (In fact, you'll find that those tubes are *still* not expensive today - it's just that they're perceived to be because of the cheap modern ones.)
Just to correct the impression given by the thread title, this is almost certainly a *junk modern production tube problem*, not a TA-15 problem. It's not Mesa's fault, every large-scale amp manufacturer has the same trouble - you can only test tubes so much, and they can still fail afterwards even with rigorous testing. If the quality isn't in there in the first place - which it isn't, on any tube currently made in the former Communist countries - then testing simply weeds out the already bad ones, it doesn't improve the others.
