Here is 1 channel of a blue hawaii running on +/-100v instead of +/-400v,
with a single 6as7 dual triode for balanced output instead of a pair of EL34.
The tube current sources have been increased from 20mA to 75mA, resulting in a grid voltage of about -30 or similar to the original design
Feedback resistors reduced to drop gain from 500-1000x to about 20x
into a high impedance load i tested 1khz/50Vpp out at .008% thd
in order to meet "today's standard" of 15W into 50 ohms balanced, I'm estimating 16x 6AS7G tubes per amp?? Sennheisers would work nicely with a baby blue Hawaii but IDK of any protection circuit that could be trusted to run no transformer or capacitor coupling into a dynamic or planar headphone!!
That's why I think this may be a better project for the purpose of phase splitting/inversion, voltage gain, and buffer driver. For example, Kevin's 'tube input unbal/bal' board with the 6922 is basically the first 2 stages of all of the SS/Hybrid e-stat designs at lower voltage but a 6922 itself can't do the job completely if the job is to get to a low distortion 100Vpp balanced output to feed Uber-buffers to get the 15W into 50-ohms
but you could use this baby BH type design, such as this with a single 6AS7 per channel or maybe 2x with the sections paralleled for more power, if wanting to have the ability to drive "Hi-Z" headphones directly with a very very good protection circuit, or maybe more realistically, a large film cap