Just a thought on that active tone control circuit. You are changing the phase of the input signal by quite a bit. Any tone control circuit that is for the most part made up of resistors and capacitors will change the phase of the signal passing through it but within approx 90 degrees. With the active tone control shown the mid range frequencies at the output of IC2C will be 180 degrees out of phase from the treble from IC2B and the bass from IC2D. So quite a bit of phase change from the original signal. It could be interesting to invert the signal from IC2C with a unity gain inverting stage before connecting it to the summing junction of IC1B to see how that affects the tone.