Reduced gain in the lower frequencies is a feature, not a bug, of the original TS808 circuit. It was meant to keep the distortion from getting muddy. The origin

Reduced gain in the lower frequencies is a feature, not a bug, of the original TS808 circuit. It was meant to keep the distortion from getting muddy. The original clipping stage includes a little resistor/capacitor snubber that rolls off frequencies below ~720Hz.