KT-88 KT-90 KT-100 6550 which has the best bass?

probably KT90, that's what I've found. EH or Ei makes decent KT90 tubes. There may be others.

It depends how you describe best bass. In my experince with a Graaf tube amp, the 6550 was a warmer more musical sound but the KT-90s were more dynamic and punchier, but lost the magic in the mids. Could be the brand of tubes used also.

Happy Listening

I went thru the 'tube flipping' phase several years ago. In a c-j premier 11, I replaced the 6550 with a quad of the the kt90 series. Yes, I had some extended bass, but lost some sound stage. I really recommend using the tubes recommended by the manufacture of the equipment. The sound will be different, but not necessarly better!

I'm with Bigkidz. It depends on how you define "better bass".

I have KT88 and KT90 tubes, and consider the bass of both to be excellent, though quite different. The KT88 is punchier, warmer, and bloomier in the midbass by a significant amount, and not as extend. It is more classically tube. The KT90 is more evenhanded, leaner, taut, and extended. It's kind of like comparing the tube and solid state sounds. KT88 tube, KT90, more like solid state.