I have an UK Amperex (so I understand that it is a Mullard that was manufactured for Amperex) 5AR4 as a rectifier tube right now (but I don't know if it would be a F31 or f32), but I still have a couple more to try out. This isn’t a bitch, but Cary could have put an access panel in the side of the power supply to make things easier.
I have been going back & forth with some RCA black glass VT 231s and a pair of vintage Ken Rad 6Sn7s that a very generous person bestowed up on me in the balanced input sockets and both pairs sound fantastic in those positions. I’ve got 4 more of the RCAs in the other sockets (no headphones yet) . . .
and . . .
you are absolutely right--it sounds so much better now than with the stock EHs!