Think of the OFs as on/off switches. With both OFs off, R20 + R21 + R22 determine stage bias. They add up to about 38k, similar to the 39k Soldano cold clipper stage. With OF2 switched on, the bias is set by R20 + R21 (about 10K) for a more modest cold clipper bias. With OF1 on, bias is set by R20 (1.5k), making the stage center-biased and therefore a clean or symmetric-clipping gain stage. So V1b can be clean/symmetric, or a moderately cold or very cold clipper. Since the stage gain is fixed, clipping behaviors can be switched without noticeable volume changes. This stage can change the character of the amp significantly and is a novel circuit that I'm testing in the amp. This stage works as intended, but it's not all that useful in this amp since the main OD distortion comes from the next stage and I mostly keep V1b in the clean setting.