Pretty batch/production specific IMHO... I know a whack of manufacturers have gone with JJs in the last few years due to cost, consistency and durability. Before that, some were going with Ruby (rebranded Chinese, Shug, Sino, who knows...). The tubes that come stock from a manufacturer are usually chosen after careful consideration of fail-rates and overall "quality of sound". They likely won't stock an already expensive amp with another $500+ in glass as that just adds to the amp's retail price, so it's left to the end-user purchaser to work with what they get.
I've had Gold Lion 77s in a number of my amps, and they've sounded fantastic, and they've lasted (in 2 of my amps) an i
ncredibly long time (we're talking YEARS...!!). And these amps spend their time 50/50 between bedroom levels to fucking cranked and everything in between. I often go full-retard on my amps to really test them out - 3 o'clock or more on the MVs, and the same or dimed on the channel volumes....just to hear how everything's "fitting together" - are the tubes crackling and cackling, any artifacts floating about, how's the hiss, how's the attack/feedback/microphonics. I play them at these levels quite often - WAY beyond what a band context would accommodate, but just to know "they can" and "without issue" run at these levels.
Tangent - sorry. The GL77s are pricey, but have sounded great and lasted a long time in the amps I've had them in. NOS Winged =C= are great if you buy from a reputable seller
https://www.thetubestore.com/winged-c-sed-el34. And I've heard great things about the New Sensor reissued Mullard and Svetlanas.
Recent Diezels have come stock with JJ77s and they sound great and haven't given up the ghost just yet. When they do? I'll likely opt for one of the aforementioned brands...but so far, no issues.
