Just a suggestion, maybe some investigation about if slew rate can be accurately be simulated with LPF? Like if we just use tl072 and increase the cap between in- & out pins. That seems to be the DRV’s workaround, I heard that piece was designed by the guy from Bondi Effects. Of course DRV isn’t trying to be a pure rat anyway. Not sure how faithfully that mimics the slew rate (or if it really matters that much). I guess I could set up a test by building the circuit both ways, but was curious if there’s a technical perspective.
Slew-rate limiting is a non-linear process and unfortunately cannot be simulated by wrapping a filter around an opamp. When an opamp is driven beyond the linear region, all sorts of things happen that may or may not contribute to the desired tone.