On my Sonic Frontiers Power 2 I took out the 6550's & replaced with KT 88's. The improvement was dramatic. The Kt 88's were Tesla & then I tried Svetlana's. The Svetlana's are, in my system, a bit more musical than the Tesla's. But not really significantly. The only way to tell is to give them a try. If you don't like them sell them on this web site.
Good Luck.
Good Luck.