Hi @TonyK , very happy if you'd like to post about it on other forums, and put a link to them here so we can follow. On the last clips I posted, from -7db downwards there was no change in tone that I could see or hear, but from bypassed then to -3.5 and then -7db ( ie the first reactive setting), there was some relative ups and downs in response plots, but no general trend. The consistency of the tone comes from the circuit arrangement and also the specific component values. It looks like we have hit on a sweet-spot that works well with a range of amps. But, to arrive there, there's some luck and judgement involved because I don't actually have a lot of data on which its all based. I only have two amps! The key info relating to the amps is the small-signal output impedance, and its measured using a consistent, non-distorted input tone, feeding a resistive load, and recording or measuring output level as that load is changed. Its actually pretty simple to do with minimal kit. If anyone is interested to run such a test, I'd be grateful and can discuss further how to do it using whatever equipment is at hand.