KT-77 vs. KT-66

as a generality, kt66/6l6 tubes are basically a weaker (less powerful) el34 -- they will usually sound weaker/slower in the bass, and sometimes a touch more ’beautiful’ in a classic tubey manner in the lower treble, mids and upper bass

but obviously, tube makes within a type, can have sonic differences that may overwhelm that across tube types (a gec kt77 vs a shuguang el34 may would sound more different than a modern russian sovtek kt66 for instance)

just remember, flexibility in use in a piece of gear (especially tube gear) usually trades off peak performance optimization

even with true auto bias now offered in some modern tech tube amps, if a designer/engineer truly optimizes his unit for a certain sound and certain performance parameters, you will hear what is intended in an amp that is specific to a particular, specifically chosen power tube (think audio research amps over the years)

power tube type selection, matching to its driver stage, and output transformers is key as these elements together provide a chosen performance envelope into whatever is anticipated as a range of speaker loads

the optimization ’point’ can be loosened into an ’envelope’, which buys you something (the option of running various types of power tubes) but it gives up something too

think of it in a photography analogy -- given a certain aperture and light condition, there is a specific focus setting that gives the sharpest image at a certain object distance... crude auto focus or fixed focus cameras still let you shoot the picture, but what is captured becomes necessarily murky compared to what is optimized