EH 6550s or Svetlana 6550C

EH's will have more high end energy. Ditch thoughts about Svetlana 6550's, they are Sovteks in desguise.

The SED "winged C" (Old Svetlana's) are IMHO far more neutral in tone and very rugged. The only caveat on the SED's is to be sure you get those with rectangular holes in the plates, not the really old ones with round holes, AND, since there have been a few 'hot' ones you take less of a chance of tube run away if you have an amp which uses manual bias as opposed to those with auto bias set ups or limited bias range. I've had one such tube I used in a Primaluna amp which has auto bias and the amp had to be sent in for repairs to the auto bias circuit.

Original Tungsol, Philips ECG, GE 6550a and SED-C are worth pursuing.

Just say 'no' EH and Svetlana (though EH makes a good KT88.)

I'm not familiar with the Tungsol remake.

I put some 6550C-SV in my CAT amps and had great results.

I preferred KT88's in the SF Power 2 over 6550's. The 6550's might have been more neutral, but the SFP2 is inherently quite neutral and benefited IMHO from the hint of warmth of KT88. YMMV.

Keep in mind and please disregard all my comments about kt88=6550
Richard Gray showed me in his Sylvania Tube text book,
a  kt88 is NOT equal to a  6550. 
Close but no cigar.
They have  slightly  dif specs. The 6550 has a tad more power handling, spec #'s. 
I have Svet 6550's.
I have ohh 400 hrs on them, , Next year when  they  have 2500+ hrs  I plan to try the EH 6550s.