Cayin CD-50T: Your favorite 12AU7 tube

Just purchased a Cayin CD-50T from a fellow Audiogoner. So far I've tried a RCA 12AU7 clear-top, Mullard 12AU7 long plate, and a RCA 5814 command series black plate. I like the clear-top for it's brighter presentation, as I'm using it with a Cayin HA-1A tube headphone amp. Any additional tube suggestions? Also, I use the metal shield provided with the Cayin for the tube to secure it to the socket. Anyone know if that causes overheating of the tube? Thanks!