Best combination for me by far was Tung Sol round plates in the preamp/gain position and a pair of Sylvania 6SN7W in the driver spots. I preferred the tall bottle black base over the short bottles and the metal bases, but they all sound great. Tung Sol is a very nice gain tube, and the Sylvania 6SN7W impart some visceral power to the representation that I think the Cary 300SEI sorely needs.
For output tubes, my favorite is the AVVT 300B. Emission Labs have a pleasant, laid back presentation, and TJ/Full Music/Sophia mesh plates are even more laid back and mellow. KR's 300Bs have a "cleaner," more solid-state-like reproduction in the Cary 300SEI. JJ tesla's 300Bs sound different in the Cary depending on when it was made (JJ revised their design several times). EH is a respectable tube for its price, albeit a little grainy and unrefined. Western Electric has incredible mids (no surprise here) but in the 300SEI, you might find the bass lacking.