Bass and treble are controlled by the same amplifier, IC4 and IC13. But also selected by the same relay, RE1 and RE2. So, check component values of C8, C9, R17, C33, C34 and R55 (4 x 100 nF and 2 x 1 kΩ) and P3. An identical short circuit or missing contact(s) in both channels is unlikely. Beautifully build! Looks good.  

Hi Joost, Which software did you use for schematics and pcb? Thanks

More pictures...

- Made new casing with the 3D printer, in different color - Made one support for outdoor fix, with external solar panel and add some recharge from the sun. Fixing it on a window outside, we can see imediatly for air quality only by looking the LED. Very very convenient... - Adjust the total thickness of the casing to be able to add a 1200mAh battery, giving more than 2 weeks autonomy with 20min measurement period. - Made a new PCB to correct some mistakes on the first one, and with a graphic display connector possibility (but SW not yet develop) - I put now 4 devices at home, one living room, one outside, and 2 in the bedroom. I launch the aircleaner with it is bad status. - Made a simple aircleaner with a portable fan and HEPA filter. It is enough to clean the air of kids's bedroom within 1h. 

Hi Joost, thank you for the reply and information