Yup, a 12AT7 in the PI really smooths out the OD and also seems to take off the upper edge off the rhythm (not that it really needed it). This was my favorite change in all the tube swapping I did on this amp.
I also put a 5751 NOS RCA in the Ryhthm channel (can't recall off the top of my head, V3 I think ...at work right now), and the mild gain down smoothed out the clean channel, but also made the volume master a bit more "usable" in that I could actually turn it beyond 2 without it going from "whisper" to "loud."
Somewhat more subtle a change was a NOS Mullard (12AX7) in the V1 ...I was hoping for more of a difference here, but frankly am unsure if there's a real difference.
And in the reverb (V5), an AT7 creates a bit of pre-delay between the wet and dry signal. This is hardly noticible with OD, but a very nice improvement in reverb tone in the clean channel if you chord or note with lots of spaces in between ...more "Fendery" reverb in its slight increase in spaciousness.
Power tubes are Winged C 6L6s: bold and tight bottom, clearly defined mids, and smooth top with zero brittle or harshness to the top end. I like these.
Finally, do the "output volume-snip the resistor" mod. Check the archives in the "amp tech" section ...simple 10-second job once the guts are exposed, and made the output knob MUCH better behaved in low-volume situations, and also seemed to sound better at "bedroom" volume. The latter may not be that important, but how much better the knob's taper feels is significant in and of itself. Hope this helps
Edward