Quick tube story with Ref 2 Phono SE.
 

Long story short:

Noticed distorted audio when powering on my Ref 2 Phono SE. If I let it warm up an extra 15 minutes after the display stopped flashing, the problem went away.

I called ARC and despite only having 398 hours on the unit, they recommended I try swapping out the 6550 in the power section.

I placed an order with Tube Depot for a new Svetlana 6550 and as long as I was doing power tubes, I got a replacement Sovtek 6H30 as well.

Swapped out the 6550 and… still distorted. I noticed that the power supply 6H30 wasn’t glowing as brightly as the four used as preamp tubes in the audio chain, so I shut it down and put the old Sovtek 6550 back and swapped out the 6H30 in the power section.

Problem solved!

So, just noting the issue for anyone else who might run across it.