The other design I made was a lot more complex with a lot more interrelated parameters that were difficult to coordinate into a working whole; you would change one thing and it would make 3 others into unusable values. I did finally get the other design into something that could work, but I knew I could do better with something simpler.

This is that design. I still need to do the power supply and calculate that little interstage capacitor between the tube stages (1uF is not correct). It looks more complex than it really is. The group of resistors, LEDs and PNP transistors simply make up a constant current source to feed and regulate the triode bellow. The next stage is a basic self biasing cathode follower. I would like to put a constant current sink in for R3 but I will leave that for a design upgrade down the road.

The potentiometer under the second set of triodes is so I can play with and find the perfect value for a 5687 cathode follower as they can be finicky and need fine tuning for the optimal value. That will later be replaced by just a normal resistor of the proper value after prototyping.

The power supply is a LOT smaller on this one. More current draw (15mA+40mA), but a lot less voltage (285V) which will make the PSU design easier. The PSU design will be really similar to the other one though.

According to my calculations the output impedance will be a tiny 43 ohms (roughly) and will supply 3.7 Watts to a 32 ohm impedance headphone load.