U1a is not a clipping stage, it's an active bridged-t filter. Bridged-t topologies provide 2nd order RC filtering, and this one is a high-pass. Like any filter, when placed in the feedback loop of a non-inverting op-amp stage, the effect is reversed, so it becomes a bass boost.

Unfortunately, bridged-t topologies are not simple to analyze, and predicting Q's and frequencies requires a non-trivial amount of math. (For non-math people, that is. As a physicist, this should be a pretty low bar for you. It's not like calculating Zeeman splitting or deriving QCD or anything. Decent starting places, if you're interested, are

here,

here, and

here, but it's unlikely you'll find any lookup tables or online calculators for this. Note that most of the practical guides and online tutorials discuss the low-pass bridge-t, which has the caps and resistors reversed from this high-pass version.)

If you want to mod it for a different corner frequency, the simplest thing to do is to sim it.

FYI - The high-pass and low-pass filters can be used simultaneously to form a bandpass filter. This is how some wah pedals work. (High-Q notch boost with variable center-frequency.)