Grab a quad of each and spend the weekend comparing them.After all,who better to evaluate the sound than the one who will be living with them.Keep the ones you like best and sell the others.Just a thought.Good luck.
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It is hard, if possible, to give you an intelligent answer. Differences between some brands of 6550's and differences between some brands of KT88's can be as great as the differences between the 6550's and KT88's. Generally speaking 6550's are more linear and KT88's are more colored, but there are a lot of exceptions to that generality.
I think you would get more meaningful responses if you were to indicate exactly what about your present sound you wanted to change or improve and identify the brands of tubes you are now using.
If you can't do that you are flying in the dark.
I have an amp coming this weekend that uses EL34, 6L6, KT66, KT88, etc. I read up over at the Asylum and there are a few folks raving about the 6N3C-e (or 6p3s-e, depending on the alphabet (Russian).
The amp comes with 6L6s, so I ordered a pair of EL34s and a pair of 6N3C-e. Jim McShane has the 6N3C-e for about $30 delivered in the US, so not much of a risk at that price.
Something to think about.
Generally, KT88 have more body/meat in the sound, but also found them much musical. Personally I compared NOS 6550 TUNG SOLS Black smooth plated and NOS GEC KT88. I also tried NOS 6550 Sylvania's, but didnt liked them that much. After all, it would be your personal choice, but after GEC KT88 nothing been better for me. Dont forget about pre-section, which is also very important, which have a huge influence in the whole. My amp was PrimaLuna Prologue Two
do you buy your tubes PL(platinum tested)? I have been told that is the only way to buy. it is 3 burn inns and tested after each i believe. I have been emailing jag music for pricing. I would require 24 + 4 extra kt88-eh-pl produced in pertersburg russia. any thoughts? I have also concidered kt88-gl-pl, which are gold lion, as well. anyone else looking for the same?