I just had another think about this, side tone adjustment is a box issue and not one of the aircraft instalation. Adjusting the sidetone should take seconds IF the box is OK.
It sounds to me as if you have got a cheap box (from E bay?) and asked these guys to fit it and there is something wrong with the sidetone adjustment inside the box.
Is this more about your buying a duff box and trying to find someone to blame rather than a well founded issue over bad workmanship by the guys who fitted the radio box that you supplied?