The Sound Sales trichannel tone control with electronic mixing was mentioned in the immediately preceding post.
What may be another example of a receiver from the same era with a broadly similar tone control was the Morton Cheney, described in this thread:
https://www.vintage-radio.net/forum/...d.php?t=159983. The bass and treble tone controls were covered in post #84:
https://www.vintage-radio.net/forum/...1&postcount=84
The circuit shown therein is thought to contain some errors, but the general arrangement is retrievable. The tone controls themselves are of the passive type, generally conforming to the Volkoff/James pattern. However, the bass and treble outputs were not recombined in the normal passive way, but each fed to the grid of its own triode amplifier stage, the outputs of which were then combined. Given that the triodes would have had a shared anode load (whether on the DC or the AC side), that I think would constitute electronic mixing. Presumably the triodes made up for the 14 dB or more loss incurred in the passive tone controls, and by individually buffering the bass and treble controls, may have reduced interaction between the two and so produced a better set of curves.
Be that as it may, the electronically mixed tone control was not an idea that flourished at the time. I suspect that the Baxandall circuit of the early 1950s was seen as a simpler way of achieving satisfactory results. It may also be noted that by the later 1970s, when the cost of active devices (particularly opamps) had dropped to the point where the constraint of device count was no longer a major imperative, Baxandall advocated the use of tandem bass and treble controls, each with its own opamp, rather than having these in parallel with electronic mixing.
Cheers,