Papa & Diezel Crew will load their amps with the most consistent sounding, best quality-per-price ratio, widely/generally available "en masse" tubes. Over the years, several brands have come stock - Ruby, JJ, etc. Some people question why such high-end amps come with such common tubes - the point is, no sense upcharging buyers for top-drawer glass when often players will replace tubes to "their" tastes well after the purchase. But moreover, it's about "quality" batches, that are "sonically" and "mechanically" consistent. Therefore all amps off the line can be tuned to sound as exact to one another as possible, and if a purchaser blows a tube off the bat, it's an easy, cost-reasonable, and readily available swap out just in the event of.
EL34s came stock for a few years, but the 77s have gained global approval and acceptance - and right now - the JJ brand is damn solid, really good sounding, relatively inexpensive, but of excellent quality tubeset. They can also take a bias-beating in the event of spikes or user installation/bias/tweak error (within reason).
Hope this helps. I've used Genalex GoldLion KT-77 for years, but my recent stock JJ tubes sound fkn awesome too. I've also used =C= 6550s, and those sounded ridiculously good, but at $100+ a pop (if yer lucky), and a slight frown from Papa for the bias heat coming off of them, I've pulled back on using them and really just enjoy my 77 style tubes (one admission, I always pop an LPS into the PI slot of all my amps - right outta the box - but that's my own personal thaaaang!!)
Peace n Powerchords
UnkleMo