I also have the PL preamp and amps. Tube rolling has been a lot of fun. As many said it’s worth it (and easiest to get an answer your question) by experimenting. That said, I found the best place to put these 12AU7s is in the preamp (4 middle spaces). The power tubes for the amps also make a noticeable difference. I’ve tried just about everything else except the KT88s. After experimenting with many, many different 12AU7s (in both the preamp and amp, and in all the spots - not just the center) I’ve found that there is a synergy with some tubes and the rest of your system. I have a brighter system so the NOS Mullards and Brimars in the preamp are a nice fit. For the amps the gain tubes I use the mil-spec GE tubes available on Upscale’s website. I find they’re just neutral and get out of the way to allow the rest of the tubes to do their thing. Most of my NOS tubes come from Brent Jesse, and as so many others have mentioned, his service is a tremendous opportunity for those of us in the hobby to get great tubes and advice at a great price.
Overall I’ll repeat the synergy thing and recommendation for experimenting in your system to find it. Also, one thing I’ve found in my experimenting is the NOS 12AU7s are a big upgrade over modern production and definitely worth buying. Brent Jesse has GEs available for cheaper than new production and they are so much better. I don’t care for the Primaluna 12AU7s and have tried other new production tubes. In contrast however, I find the stock EL34s to be excellent. Lastly, be sure to listen to any new tubes for several weeks. Don’t rush to judgement on them, they need to burn in.
