saffy, I do not have access to the Jadis so i can not tell you how a particular tube will sound in it, but i can tell you that, power wise, there is basically no difference between 6550's, KT88's, and KT90's. There is a difference sound wise which is very system dependent. Detlof's comments on the KT90 mirror mine, and dispite the fact that many folks like it i do not use it (i have a couple of used quads - just in case i change amps some day). If you suspect that the mid range distortions you noted in your other thread might have their origin in the power tubes i would get a quad of Svet 6550's and Sovtek KT88 (don't laugh too loud, they arn't all that bad, they are rugged, and sound prety decent used with the right drivers)or Svet KT88 and try them. They all sound different. Could be an interesting expiriment done on the cheap.