Hi RATs!
The reason a tube sounds good is the circuit it is in, if it ever
sounds good. Not because it was "designed" for low (audio) frequency.
Low frequency is not a huge technological leap, unlike GHz, for
instance.
I am using old TV horizontal amplifiers, 6BQ6, My music is not as
demanding as kicking the electron beam across a CRT.
But, having a ten watt continuous plate rating is a bit less limiting
when the tube is also speced for 500 watts for short periods.
It doesn't matter what the tube was designed to do. If you put it in
an audio circuit and like what you hear, it is an "audio" tube.
No rules for us fools.
If it doesn't work, blame mgmt
Happy Ears!
Al