At the risk of becoming an unmentionable name here wouldn't trying to keep the grid chokes inductance more or less a constant insure the very problem you are trying to cure? Namely that if the inductance is a constant then the load is a frequency dependant variable. Instead if the inductance is high enough to swamp the load resistor and tube's internal resistance at low hertz and then falls in a more or less linear manner so that the actual impedance the tube works into is a constant wouldn't that be best? Off the top of my head 1,000 henry at 20 hertz would be 125K but at 20kHz only 1 henry is needed to have the same reactance. Of course there are more things to consider than just the inductive reactance but I think you get the gist of my question. Now I have no idea just how the reactance falls as a function of frequency but offhand to keep a constant load I come up with roughly needing 1/50th @ 1kHz as at 20 hertz and 1/1000 @ 20kHz as at 20 hertz.

Russ