Why are JJ Electronic R.F. preamp tubes preferred over A.F. tubes in guitar amps?
I see nothing in the more extensive data sheet for the ECC83S that makes it any more suitable for RF than AF. In fact on the link I've provided, Mullard say this: -


  The ECC83 high impedance amplifier triode was designed specifically
  for audio use and has a high amplification factor of 100.




The ECC83C is two triodes in one package and here's another good use of it at audio frequencies, a balanced microphone preamplifier: -



Here's a circuit that uses the ECC83S as a differential amplifier feeding a push-pull stage: -



Neither Mullard nor most of the circuits hint that it is an RF only device.

I can only conclude that it is marketing hype or a mistake that lables the ECC83S as "RF". In fact if you follow the ECC803 (RF) link it takes you to a data sheets that shows this: -