I have both the JJ KT88 and Ei KT90 tubes in my home.
While I prefer both to the 6550 tube, it depends on what your priorities are as to whether you should again make the jump.
The KT90 will take you back in the sonic direction of the 6550. More deep bass, less punchy midbass, more neutral sound, and a bit more top end extension. The JJ KT88 have a seductively punch midbass and sweet sound overall.
I wouldn't be surprised that if you very much liked the improvement the JJ KT88 have provided, you will not be so in love with the KT90.
While I prefer both to the 6550 tube, it depends on what your priorities are as to whether you should again make the jump.
The KT90 will take you back in the sonic direction of the 6550. More deep bass, less punchy midbass, more neutral sound, and a bit more top end extension. The JJ KT88 have a seductively punch midbass and sweet sound overall.
I wouldn't be surprised that if you very much liked the improvement the JJ KT88 have provided, you will not be so in love with the KT90.