I'm curious about this thread also. I have a new PH 9.0 and am enjoying it.
I've listened for a little while with the supplied tubes. They two 6922 variants are 6H23's, but I'm not sure of the brand.
I've swapped in some Orange Globe Amperex 6DJ8s from the late 60's from Herleen. I felt they did make an improvement.
I've been considering trying others, but I only have the Orange Globe's in my collection, so if anyone has positive experiences with other brands in the 9.0 I'm interested in hearing about it.