The older Russian 6P3S-E are a good choice for the P2SE, but I found it hard to discern any real noticeable difference between them and the Sovtek 5881's. They appear to be an older vintage of basically the same tube. I think the earlier Sovtek 5881 had the same dimple getters as the 6P3S-E, whereas later versions had ring getters. Another nice tube for the P2SE is the new production re-issue Tungsol 5881.
I had a P2SE for 16 years and not a day goes by that I do not miss it.
