Hola Mike.
I must understand that you will receive 6550 manufactured by KR?
If so, it would be great news for all of us who use 6550 NOS in our Audio Research Pre Amplifier.
It is increasingly difficult to find them.
I'm not sure of the question here. Are you asking me if I have tried the KR Audio KT88 (they don't call it 6550) in the ARC preamps? No. I haven't even tried them on anything yet. I will. I recently took three huge boxes of very rare NOS tubes on trade. I'm busy sorting through those.
For the ARC Preamps, I have a stash of Winged-C 6550's I need to try.