The JJ's arrived today. Played for a few minutes with the stock tubes installed before swapping. The JJ's are a little darker which is not a bad thing in this amp since it is so bright and mid-heavy to start with. The bass is tighter, the top end is smoother (glassy, perhaps?), and there is more clarity. These are very musical tubes, IMO. There was a certain frequency that sounded harmonically unrelated to my ears with the stock tubes that I couldn't seem to dial out. I don't hear it with the JJ's. For my tastes, I think they are exactly what the amp needed to polish it off. The tube rattle is gone as well so that confirms it was one of the stock tubes. Of course the difference between the tubes is not night and day, but they are subtly noticeable differences which I think improve the tone.
Haven't touched the preamp tubes, but I don't intend to go down that path again with this amp. I'll stick with the stock JJ's there as well.