There are some challenges like this in the guitar world, but the 6L6 is still (mostly) a well built tube, and many of the amps do not run these tubes as hard as the KT-88 (which has a unique tone and overdrive character at high voltages because of how the screen current interact with the plate current and the harmonics that this introduces. The driver topology is also quite different, actually much of the power amp circuit is quite different compared specifically to the Strategy/Prodigy.
Over the years, I have seen consistency issues (in a variety of applications) with EL-84's, much of it mechanical where after a few hundred heating/cooling cycles there is enough change in internal structure that they will make some mechanical noise. The is 100% a tube issue, as it occurs regardless of the amp the tube is used in. There is a little bit of this in the El-34 as well, but nowhere near the extent. The 6V6 is a pretty stout little tube, relatively trouble-free but like the EL-84, is limited to lower powered applications.
The Subway series was never intended to go after the same tone market as the Strategy/Prodigy, it's just a way different tone. There are plenty of players who do not care for the Stratigy/Prodigy, and who prefer the Subway, or the Walkabout, but the Strategy/Prodigy was never intended for that player to begin with. It's more like the Carbines, in your face, brash, knock you over your head with little politeness hard rock and metal amp.
Hope this helps.