before the advent of modern day russian engineered kt120’s kt150’s and the like, kt88/6550 tubes were the more powerful, most linear strongest pentode/tetrode power tubes, with max plate dissipation about 30-40% higher than earlier el34/kt66 types
as such the kt88 variants offered the greatest linearity when called on to deliver near max power in real world speaker-driving, a level of which is much higher than older weaker el34’s 6l6’s kt66’s as per the above... so what happens in ’typical’ transformer coupled tube amplifiers, using each type of power tubes in pairs, quads, and so on, is that kt88s would provide noticeably better dynamics, more controlled tuneful bass, more extended treble, cleaner transients -- whereas the weaker earlier tubes variants in the same circuit or application (each biased correctly, it is assumed) will sound more mid-range centric, have a more smooth-over, ’romantic’ treble and somewhat slurred transients, have warmer tubbier slower, not-quite-as-deep bass -- this leads some tube amp aficianados to say that the older weaker tubes made for a sweeter sound, where later stronger tubes made for a leaner more solid stateish sound -- all is relative of course...
as generalities go, for what they are worth, this is what one would expect...
