Audio Research Ref5 KT88

Is there anyone replaced the 6550 in the Ref5 or Ref3 or Ref cd8 for a KT88. I have a few nos GEC kt 88 from the fifties and the sixties. I replaced one in my Ref cd8 a few months ago and in my opinion it has more detail, finer, opener and smoother.
I have the REF 5, but have not even thought about changing to the KT88. I did change my Cayin KT88 to the Tungsol 6550. I liked the 6550 better in that amp.
I do not know off hand if all of those components use tubes in their power supplies, but if they do, it is the 6550, and for a reason. While the KT88 is extremely similar in specifications and design (a beam power tetrode), the designer chose a 6550 for deliberate reasons as the 6550 is typically more robust in that it draws and handles a higher current capacity; which for a power supply, is a great attribute in many aspects.

I would contact ARC tech support for your inquiry and see what they have to say. It may void a warranty if you do it or do anything at all since it is in the power supply (if applicable to that model).

You would also benefit from finding out the exact current draw for the tube(s) you intend or desire to replace as a lower value may not be able to handle what the circuit is designed to deliver or carry. You may need a tube that handles 65mA of current all the time or one that can handle only up to 53mA or so. It can make a big difference in performance regardless of how the sound is impacted, or your warranty. Good success, but call ARC.
Do not do it. 6550 should be the only tube used in the Ref 5 and Ref 3. Make sure you get a 6550 that has been tested becuase you will hear it if it's noisy.