I didn't knew they were sold ready to go. I've used this kind of approach in some guitars. Anyway, I hardly ever use them or tone pots for guitars, but I like them the other way around (as hi-pass filters) for some amp circuits to keep the lo-end tight and focused while turning the volume up. Sometimes the Bass knob on amps is not a good one to solve flabbiness. Orange, Matchless or Bad Cat used these circuits in their amps.
RC and LC filtering circuits were widely used for guitars in the '50 and '60s tough they dropped popularity by late '60s. Fender used these filters in the early Esquires, or Jaguars and Jazzmasters. Gibson had the Varitone. It was also widely used by German makers. The Schaller or Star pickguards have this kind of filters on-board, or from the DDR, Simeto stuff has them too. This Musima Record has two filters (I cannot see more that two capacitors there). One is an RC high pass filter paralleled to the bridge pickup (Switch position 3) thinning a the sound a bit. The other one makes also an RC filter but this time connected in series with the neck pickup acting as low pass filter, leaving a really muddy, dull and useless sound. A severe treble bleeder (switch position 4). LC (inductor-resistor) were also used, but inductors are much more expensive than capacitors, so only a few builders used LC resonant filters for some "exquisite" products. Examples for this are the tone circuit of the Ampeg V series amps from the 70s or the Gibson Varitone sometimes found in the ES-335 guitar family.
I have to say that if I ever want to filter something I prefer to make it this way, fixed or selectable fixed RC filters. I've never favoured conventional tone pots. Most of the times I rewire the pots as fixed resistors to turn them into fixed bleeders, or just completely disconnect them from the circuit.
