This letter to WW 1949 April made a prior claim, dating from 1939, for the James tone control circuit, with Volkoff as developer. Thus it predated the Voigt tone control. Probably James was unaware of the previous American work.
In Radio & Television Engineer’s Reference Book, Third Edition, 1960, (RTVERB III) the section on sound reproduction and distribution noted that two kinds of treble and bass tone controls were in common use, one being passive, and the other being the Baxandall.
from R&TVERB III pp.37-20,21,22.pdf
The passive tone control circuit was essentially the James/Volkoff circuit, with the additional standoff resistor. Perhaps the RTVERB editorial staff avoided naming the passive circuit to avoid getting into any issues about who developed it. On the other hand, the Baxandall case was clear-cut, with, as far as I know, no other claimants.
Cheers,