I forgot all about it, probably because I quickly gave-up on finding dual 250k/500k pots.  PS do com reader Alekos Gavriilidis from Thessaloniki Greece went ahead and built it anyway, using separate pots instead of dual-gang pots , and found this to yield useful results.  Alekos writes:

“… i’ve just finished the project (EQ TUBES 6SL7). Im sending you photos of this project. I used 4 tubes 2 in parallel for every channel.  Now the sound is incredible!!!!!!!  P.S. I had a little noise from the filaments so i changed the voltage from AC to DC volt. Soon it will be in a studio”

Here’s his notes on the schematic:

Here’s a quick diary of his build:

…and here’s the layout that Alekos used for the panel:

He also provided plots of the action of the various controls:

Thanks very much for sharing this with us Alekos!  If you’ve had success building one of the circuits that I’ve posted here over the years, please let us know about it… your success might encourage others to do the same.