Cary 572-3

does anyone know what tube(s) Cary used to replace the 572 in their rebuilds of this model and does anyone have any thoughts about a replacement (845)?
Most I have seen were turned into 300B SE's. I believe Cary has a fixed cost for this mod. Sad that the 572-3 is no more. You can't substitute an original 572 because the pin connections are different. Have you exhausted every source for these tubes? If I had these amps I would get my hands on all I could find before they are totally unobtainable. I heard they don't have a long service life.
Thanks for the reply, I assumed that a 300b might be the obvious choice as the real estate allows this easily, but the 300b is a vastly different sound to the 572. I have about 6 572s so no current problem, but they do have a limited service life. If I was to change would probably go with 6SL7/6SN7/VV32 (I have a few of these too)