I disagree with Doug. I doubt he's ever played a Chupacabra or messed around with tubes in one.
The Ruby EL34BSTR is a bit bright and clinical. Mullard... meh. Not for this amp in particular.
Ruby AC7 is pointless in the PI section. A long plate tube like the Sovtek will be best. No need for balanced sections either.
You can go with whatever you want, but I've had a Chupacabra and a Yeti for like 8 years now. And I've built 4 of them. I've used all the tubes out there. For ME... and the type of music and sound I go for, I really like the JJ E34L power tubes (or RFT) and the preamp tubes I mentioned already.
The 6N4-J is awesome for amps like this. They are affordable and don't have all that NOS snake oil going on. Doug is recommending tubes he sells. Because he sells them. He has a business to run. The Beijin 6N4-J is much better than the Ruby AC7, which is the 7th generation version of the Beijing. Sort of. You want a long plate in the PI. The Sovtek is cheap, reliable, and works great. This is all my experience. But do whatever you think works.
If you do some research you'll find the Sovtek LP is recommended in lots of hot-rod Marshalls. And the Beijing is a great secret weapon for V1 or V2. The RFT is very vocal and has a unique mid-range. But it's slightly rolled off highs, which is good in V2 of the Chupacabra.
