EL 34

I have a Leben CS600.
I believe the KT 77s would be an option.
I would favor a smooth top end

@mpomerantz a jeffsplace review replacing the stock electro harmonix EL34s with Sophia Electric EL34 and KT88s, on the Leben 600X. Gives an idea of what room for improvement might exist over the stock EH for your type amp.  Other member feedback comments below the article too, fwiw. https://jeffsplace.positive-feedback.com/leben-cs600-cs600x-tube-rolling-with-sophia-electric-vs-ele...

RAM tubes :-) Roger Modjeski but still going strong w super matching and testing, $45 gets you a killer valve

in my experience

ranked by overall sq (including midrange beauty) - we are talking old stock tubes here, not the new/modern day russian/czech/chinese tubes

best
gold lion gec genalex kt77 (uk made)

close/next best
mullard/amperex el34 (uk or dutch made)
matsushita el34 (seam top) - (japan made using mullard tooling)

close/next best
svetlana sed winged c - russian old stock st petersburg plant
tesla yugo el34 brown base yellow print

not as close, next best
philips/national east german thin bottle dimple top el34
yugo ei fat bottle 6ca7

amps i have/have had in which i developed this ranking - vac pa90c, air tight atm-3, primaluna hp prem dialogue, linear tube audio zotl40 reference, marantz 8b, cj prem 11a xs

When anyone tells me the TOP is a little harsh, I always ask if they can BIAS a little lower. The valve is breaking up to soon.. 50% of the time it increases the mids and highs and usually increases the  peak to peak by a bit. What I do notice is low volume playing (VERY low) is not near a clear or pleasant.

Swapping a signal valve or two always smooths out the rest. Telefunken or Mullard Blackburns will do it..

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 Electric EL34 and KT88s, on the Leben 600X. Gives an idea of what room for improvement might exist over the stock EH for your type amp.

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I'm just assuming you can use a 6550 too. There are GREAT 6550 (W) Russian surplus and new that are wonderful valves.. That W means they are MiG tough.. That's pretty tough.. They are a thick black plated valve.
Super stout banded/base.. Just make sure YOUR vendor checks the factory match.. Their QC for boxing valves sucked.. CHECK for matching.. Mids and highs.. 6550s..:-)

Regards