A possible reason for an electron tube at 5-10μA sounding so good as mic booster amplifier may be...

Not only the interaction of the increased anode inoedance with the output transformer primary.


But also the output transformer primary wiring resistance (effectively in parallel with L) possibly creating a more gentle than 6dB per octave low cut or high pass filter.


See recent previous posts including the 2DV4 Nuvistor electron tube at 1.3V.