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.
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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.