From the Quad 44 we can go back to the Acoustical QA12/P of 1947. This had a tone control that predated the 1949 James passive tone control but was generally similar to it.
It included a standoff resistor between the bass control potentiometer slider and the output. Also, capacitor C10 was placed following the treble control potentiometer slider rather than between the lower end of the potentiometer and earth. The net effect was probably the same, in that the capacitor was there to provide a high impedance to bass frequencies, so that they were essentially unaffected by treble control operation. A similar, but not identical variation in capacitor position was also seen between the two variants of the Baxandall control.
Evidently there were several separate but similar developments going on in the field of in the passive tone control circuits back in the 1940s.
And whilst one may find mention of the 1939 Volkoff tone control, there do not seem to be any actual details available. I suspect that where it has been mentioned, the source has been that WW 1949 April letter.
Cheers,