Not a Sphinx, but FWIW when I had a Rogue RP7 (driving a ST100 I still have), my favorite 12AU7 type in the primary position was the venerable Amperex 7316. It was the most dynamic, holographic, with just the right amount of "sparkle". The Amperex 7316 is difficult to find these days and commands quite the price. For a "fatter" sound, but still plenty of top end air and a wide bandwidth, RCA black plates did very well. These tubes were sourced from Brent Jesse. I tried many 12AU7 combinations and spent entirely too much money doing so, but I learned a lot and it was fun. Honorable mentions go to the Ken Rad 12AU7, which is a very unique and beautiful, almost chimey (in a good way) tube. I did not like, and TBH have never liked Mullards in my equipment.
