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.