Suggestions for replacing the 6550 tube in the ARC Ref 6.

I find the stock Sovtek 6550 used by ARC to be rather good in most respects, but was looking to find a substitute that might possibly be better. I noted some comments praising the NOS Tung-Sol black-plate and grey-plate 6550. I contacted Andy at Vintage Tubes about these and he said he found no significant difference between the black-plate and grey-plate 6550 Tung-Sol. I got the grey-plate Tung-Sol, and after 50 hours of break-in time, I was very disappointed in this tube. It had more detail, a tighter, deeper bass, more extended highs and was more dynamic. However, the midrange was totally sacrificed—not realistic and what will give you “listening fatigue” after a brief period of time. Stringed instruments and woodwinds were bright and edgy, and piano and vocals were particularly bad. The original Sovtek had more body with a sweet, warm midrange that was altogether more musical. So—if anyone out there agrees with my negative appraisal of the NOS Tung-Sol and has any suggestions for a good 6550, let me know. Otherwise, I will be content to leave well enough alone. As a cautionary tale, you do not always get what you pay for; more expensive tubes are not always better. People who claim one brand of tube is better than another may not have the same taste as you do.

I have tried a bunch of these over the years. Now I just order another set of JJ. When one of the most affordable is also one of the best sounding, what's not to like? 

I had seen references to differences between the black-plate and grey-plate on this site. Andy of Vintage Tubes said he does not think there is much difference in the two. He also said the NOS grey-plate had a good midrange, so I certainly would not trust his judgement as far as Tung-Sols go. In regard to the recommendations for the JJs and the Svetlana 6550s, have you heard the stock Sovtek 6550—and, if so,  can you tell me what the difference is in these?

The tubes that came with your amp can often be the best sounding tubes. When you buy different tubes the sound will change, sometimes for the better and sometimes not.