KT66s in place of KT88 tubes

specifics always rule the day 

generalities only lead one astray...  

poetry to live by 🤣😂

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...

@dogberry

if you look at the quad website, quad ii/40’s run kt88’s and rating is 40 wpc (as mono blocks) ... whereas quad ii classic run kt66’s and are 15 wpc as a stereo amp, so here you see apples to apples the kt88 application has about 25% more power than kt66 (5 wpc more)

old school quad ii’s always ran kt66’s iirc (there are many refurbed old ones for sale, as can be seen via a simple google search), so it is evident in the new ii/40’s they uprated the internals to run more powerful kt88s to gain more output power

i think you could easily ping quad in the uk, or a usa quad dealer or quad importer, and ask if they advise users with ii/40’s it is ok to run kt66’s as a downgrade

rule of thumb i use is never ever run a power tube in a tube amp that was not designed for it... only bad things can happen (though some amps, like primaluna’s do have variable or auto-biasing that do allow a choice of output tubes... but this is usually not the case)

all this said, if they tell you that it won’t hurt anything to run kt66’s in the ii/40’s (other than having less power), then i suspect you will get a more ’tubey’ sound with the kt66 tube ... for the reasons i described in my earlier comments