I want to use it as clean/low gain boost to get cleaner vox tone.
Is there any way in this circuit to widen the clean range of gain knob? Or - better - to reduce clipping without loosing voxy tone and volume? Changing gain pot didn't do the trick, lifting two last LEDs reduced volume drastically. Any tips?
If you want to reduce the gain in the "preamp" section, I would remove the 10uF capacitor connected to the source leg of Q2. That will cut the gain by about half. You could make that switchable so as to recover the original gain if you want.
Here's a different solution that you might try as well:
Remove the diodes that goes from the audio path to ground. (the 1N4148 diodes)
These are the ones doing the majority of clipping in this circuit.
This way you remove the harshest clipping, but you keep the tone/sound of the circuit.
Note that a diode going from the audio path to ground, creates HARD clipping = Distortion but Less Volume.
Diodes in the feedback loop of an op-amp, creates SOFT clipping = Overdrive and a bit More Gain.
So therefore you got lower volume when you removed the LED's from the feedback loop.
You will get a lot more volume output when you remove the 1N4148 diodes, but less clipping/distortion.
This will probably make it a bit hotter as well, since it will push the last gain-stages a bit more, but you will get a cleaner sound at lower gain levels.
I would start by removing the last set of diodes first, and work your way towards the input if you like the result and would go a step further...