We are still sorting out how to best implement the DC heater modification we would like to provide, as my first crack at it produced too much heat in some parts that aren't all that easy to take out and/or replace.
With the Moreplay being my first Bottlehead preamp, what aspects of sound quality are improved by the upgrades, e.g., bass, resolution, smoothness, etc.?
When I get them working, we will be sure to be more specific about that.
The C4S and attenuator upgrade will better optimize the load seen by the 6V6, and the discrete resistor attenuator provides nearly perfect channel balance at all switch positions and offers subjective and objective improvements over a potentiometer. A shunt regulated power supply will provide greater separation between the raw power supply and the audio circuit, and the shunt regulator itself tends to provide better dynamics overall compared to the same circuit without it.
The DC heater supply will knock down the 60Hz noise that's somewhat present in the noise floor of the preamp.