Yes they are, the heater and cathode are one and the same. Known as "Directly heated". Most offen seen in triodes,
but here you go.

The (I think #5) was the side connected to ground, that is the one to play with. Maybe connect a big 500K or more, pot across it and see what happens.

Unlike most of us over here, you are trying to make your amp sound "worse" but hey...

Poke around the web, there are a lot of guitar guys out there, looking for the same thing you are...Good luck...John