A distortion that keeps its spectral balance

The problem with placing a tone control

A clipper generates harmonics from whatever it is fed. Feed it a signal with the highs lifted and the harmonics land differently than if you lift the highs after clipping. Both are common, and neither lets you change how the distortion sounds without also changing how bright the output is.

Boost, clip, cut

Put a high shelf before the clipper and its exact complement after it. With the clipper bypassed the pair is a unity-gain identity. With it engaged, the shelf gain sets which part of the spectrum drives the nonlinearity while the output balance stays fixed.

shelf(g, f0, s) = ...;
process = shelf(g, f0, s) : clip(knee) : shelf(-g, f0, s);

The slope of the shelf is the interesting parameter. A first-order shelf spreads the emphasis over a wide band; a steeper one concentrates it.