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Terry, I have a Deluxe Nashville Tele like yours -- prqbably the biggest differences are the Q-Filter and an L-290TL bridge pickup. The only use I found for the original Fender tone circuit was to throttle back excess high-end harshness, which BLs don't have unless you don't adjust your amp correctly for the room -- so I seldom used it at all.

As for describing the effect, boy is that ever tough! Wired in with the resistor and capacitor, it's kind of like a two- or three-band EQ controlled by a single knob -- the entire tonal balance or timbre of the instrument gradually changes as the knob is turned. When you get the knob toward the counter-clockwise limit, the sound gets close to an acoustic, and BL assured me that if we had decent wide-range speakers (or add-on tweeters) in our amps it would be a pretty decent imitation. In between the wide-open electric sound and the mellow quasi-acoustic effect when the control is backed way off, there are dozens of in-between settings that you can use as a sort of tonal pallette to suit a particular tune or section of a piece. BL has explained to me that when you wire an inductor in parallel with another inductive device like a pickup, the electrical effect is to reduce overall inductance. This seems to "sweeten" the tone, but with the capacitor and resistor involved the effect is more than that -- the capacitor bleeds off some highs and the resistor bleeds off a bit of the entire signal at the same time the inductor is doing its sweetening thing, so the Q-Filter action is multi-faceted and, unfortunately, difficult to put into words. I think you might have to actually try a guitar with one to really get a feel for what it does. Sorry about that! :(

One very easy thing to do with a push-pull pot is to switch between using the Q-Filter a pure inductance and as the full LCR network as per BL -- just use one pole of the switch to bypass (short out) the resistor-capacitor pair so that only the inductor is really in-circuit.