I used to own the ARC VS-115 amp which came stock with Svetlana SED 6550 winged Cs. ARC later approved the KT-120 for use in the VS-115. IMO, the KT-150 ... hands down ... was a big improvement over the SED 6550 winged C. Better dynamics. Better bass control. More headroom. Just plain sounded better.
Having said that, I couldn't stop "wining" when the Svetlana factory burned down and reliable SED 6550 winged Cs became scarce. As my fellow ARC preamp and source component owners may know, the SED 6550 winged C was used in the power supply of many ARC source components and preamps. ARC now uses the 6550WE, which I believe is made by Sovtek.
IMO, I think the difference between tube types is of much greater import in ARC's amp gain circuits as compared to the preamp or source component power supplies. IOW, I think the 6550WE, or the 6550 Tung Sol re-issue (also made in Russia) works just fine in the power supply applications.
Perhaps Al or Ralph could better explain why (if at all) that might matter.
Signing off for a glass of whine.