Re: Time to retube what's your favorites.
First off, Doug does a good job recommending tubes. Those Pentas are "just" selected Shuguangs, but I've had good luck with them. I like the new Tung Sols a lot, although they tend to run a bit hot in terms of gain. (You can ask Doug for hi/med/lo gain if you want.) I've also liked Electro Harmonics 12AX7s in some spots of some amps. They don't color the tone much, and that may be a good thing in your PI.
All that said, no two tubes sound exactly the same IME. I recently delivered a Super Reverb clone with a Penta in the V1 spot. I'd bought several ANOS 12AX7s and tried them there, but that particular Penta just sounded so sweet there.
12AX7s aren't that expensive, so if you're placing an order why not throw in a couple of extras to experiment with? Personally, I find JJs to be dark and a bit muddy but you might like them in your amp. A white box Shuguang 9th gen. Chinese is a cheap addition which might just make you smile, or a second Penta. Having a hi-gain and a lo-gain Tung Sol to experiment with would be good too.
Just a couple of thoughts. Can't say anything about EL-34s though.
Chip