i use and abuse of a rat and a ts9dx, the ts9dx has 4 modes to switch from gently sweet from srv to great overdriven tones.
the asymetrical clipping is an easy mod in diode pedals, the diodes cuts the summits of the signal waveform to generate the classical square waveform on time domain and high odd harmonics on spectral domain. the square could presents a perfect simmetry with the x axis ( the same value of voltage in positive phase and negative phase ) or an asymetrical waveform with one of the phases with higher voltage, to do that mod you only need to get to different diodes with distint drive voltage, even you could get one 0,7 volts diode for one phase and get two 0,7 in serial 1,4 volts for the other phase, also you could install a switch to toggle between symetrical and asymetrical cut