McIntosh MC275, which tube works the best?

I have the new version of the MC275 (with balance inputs, stainless steel chassis). I am using the 6550c tubes that I purchsed with the amp. I believe they are made by Svetlana, but with McIntosh logo printed on the glass. Thinking about replacing the 6550Cs, any suggestions?? thank you.
That amp can also handle the KT88 but check out the KT 90-type 3 from - a great sounding tube. The 90 is also compatible with that amp.
Oh no, you need vintage tubes: Tung-Sol 6550 or Gold Lion KT-88. Different as night and day!