Unwanted volume boost

Hi there!

I've completed the pedalshield for STM32 (without Jumper1 it sounds better) and I'm pretty sure my circuit is the same as the schematics.
I've tested it and the only things that bugs me (appart from the noise, because im on a breadboard) is that the volume is boosted, which cause it to clip a little bit.

With the clean guitar example, without the volume programmed, I do a direct adc read to dac write. I've tried turning the RV1 (500k) pot all the way down but the signal is still 25% louder.
I can achieve a good result by turning down the volume knob on my guitar, but it's not ideal.

Is that a normal problem? Am I missing a part or can I add a part to cancel the boost?

Thanks!