Hi, Carl.

I'll bet the KT120 tubes sound wonderful in the Premier 11a.  If you are due to replace your output tubes, this might be a better choice than another set of 6550s -- not because of power but because of sonics.

I would double-check with Conrad Johnson by e mail.  Of course, you will have to reset the tube bias via the trimpots.  If you want a set of KT120 tubes, it might be a good idea to buy them from Conrad Johnson.  Or Jim McShane.   Or another reliable and known tube supplier who matches and tests the tubes before sending them out. (The KT120 is getting a reputation as a well-built, reliable, and consistent tube.)  You probably want a matched set of 4.  Strictly speaking, you could do with two matched pair -- one pair per channel.

Having said that, I am cheap.  If you still have plenty of hours to go on your 6550s, and especially if you put them in just recently, I might wait.  You can always do this six months or a year from now.

Also, if you do replace your present tubes with new ones, you know that they will take a while to run in.

And ... it wouldn't hurt to buy one spare.  Good listening.