Best 6550's for ARC VT-100 MK3 amp- SED; Tungsol??

Just picked up the amp- I believe the stock ones are in there now- Sovteks?? Good bass; dynamics; perhaps ever so slightly 'glassy' on top at times- they could be older- not sure...

I have heard the Svetlana (SED) 'Winged-C' are good? Apparently, it's difficult to know if you really are getting SED's though, and not the Sovtek equivalent...

Also- what about the Tungsol 6550 reissue- how would they compare to above?