Which manual stated line input? You only have 1 line input on the AB, and you are trying to send a stereo signal to it. Plus you are sending an amplified signal (small amp for headphone), so you will have gain staging to contend with.
That being said, you would either need a 3.5mm to 1'4" mono, or 3.5mm to XLR male stereo (2 ends on the cable), to XLR Femal Stereo to 1/4" mono. Plugged in to input 1.