whirlwind
Headphoneus Supremus
The 6K6 is also a real pentode not a beam pentode like the 6V6 and 6L6.
But all these tubes are wired the same and can use the same adapter.
So EL34 and the 6K6 could be interesting.
One thing about the power tubes in this amp, they do not add anything to the noise floor.