The preamp is, in fact, based around opamps (for clean gain) feeding into a soft diode clipping network (2kohm series resistance, as is) into a JFET configured to clip asymmetrically.
View attachment 351768
This is the basic "building block" (with lots of room to tweak). Input is a 50mV -> 500mV peak signal varying by 50mV steps and gain almost maxed. The THD rises essentially linearly up to a point (about 6-8%), and then starts rising a bit faster (and then, of course, slowing down to a plateau). This is not exactly tube-like but it should be good enough for touch sensitivity. By increasing the clipping threshold and/or increasing/reducing the series resistance you can get the stage to clip a bit differently (faster or slower). Without it altogether the JFET clips harshly once reaching its Vgs limit.