perhaps
state amp and high
those old tube things,
Many years ago (probably 1977 or 1978) Charlie Watkins of WEM gave a talk to my university audio society. One of the questions he answered was "why not use non-linear networks to simulate the valve sound" in guitar amps.
The answer was that WEM had tried hard to do this, as there would have been considerable robustness and cost advantages. Unfortunately, they could not get the sound right, so they decided to carry on making valve amps. I think the problem was not just getting the right transfer function, but also the time dependencies like power supply sag and even valve microphonics that affect the overall sound under severe overload.
John