Help with gain tone control

Hi, i've build this 3 bands tone control, original schematic from Nuova Elettronica.
If you don't consider the first OP opamp, can you tell me how vary gain in the second OP amp? Sorry for my language...
 

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Thank you.
I want to increase gain, not decrease.
I don't know why, but i've some distortions with a gain of 7.

This is for a friend of mine, he want more gain.
I've removed R4 and used R2 - 100k and R3 - 680k and i have distortion.
I've made many others pre with OP amp but this is the first tone control.
Schematic is ok and pcb too, it isn't problem gnd or power supply.
Do you have some ideas?
Thank you for all.