When you want to use your directly heated output tube
with AC heating for the filament,
you need a good sounding hum bucking potentiometer.
Even with a DC heating the use of a balancing potentiometer
offers sonic advantages.
The Cosmos RA25Y and RA30Y are used by Sun Audio and
Kondo and are (for this application) indeed the best
sounding potentiometers we could find.
Kondo Souga amplifier