As well as distortion, Amir's tests show poor frequency response, which I'm guessing is input coupling capacitors. I can't see the size of those chip capacitors, but if the ADC chip is the CS5340 (or a reject or a clone at that price) then, if I've done my maths right, a 25k input impedance and no buffer would need a coupling capacitor of around 630nF for a -3dB at 10Hz so good for -1dB at somewhere near 20Hz. 630nF doesn't sound unreasonable for a chip capacitor, so doubly surprised at the lack of LF.
S.