I have used SED and Tung Sol 6550 in my VTL ST150 and I found the Tung Sols to be more to my liking. The Tung Sols had a firmer bottom end in my system. The SED sound was more open and airy than the Tung Sols but not by much. I felt that it was a good trade off and have stuck with the Tung Sols. I do want to try the Shuguang KT88-98 but that will have to wait a few years untill the Tung Sols wear out. Rolling output tubes gets expensive!
Cheers,
Jim
Cheers,
Jim