I have rolled Mullards, Telefunkens, Amperex PQ 6922 and bugle boys, Sovtek 6922, JJ 6DJ8.
To my ears, there was a world of difference between the older NOS stuff and the Sovteks. Sovtek were just bright and brittle. Terrible.
The Mullards were darker and softer, the Telefunkens neutral and clean, PQ similar to the Telefunkens but not as clean and smooth. By far, in this amp and system, the best sounding were the bugle boys (tubes ranged from 1962 to 64). Beautiful open detailed sound. Made the music seem present and alive. Awesome tube. Truly made a significant improvement. The Mullards and Telefunkens actually made a good mixed combination (4 6DJ8 tubes).
The power tubes in this amp are EL34. Could also roll in KT 88 and KT90 tubes. Power tubes didn't effect the sound as much as the driver tubes, but the type 2 KT90 Ei tubes were awesome. All gone now from what I understand.
I'm interested, if I get the Valhalla, of the effect of tube rolling.