The amp has balanced inputs, the airport express has unbalanced stereo outputs. You'd be feeding left and right to the differential inputs of each amplifier....the output would be the difference, L - R.
Thanks for getting back to me so quickly. As I'm a bit of a noob when it comes to balanced/differential inputs, is that a yes? Am I ok with just the proper cable and plugging the airport express directly into the amp?
What you need is a 3.5 mm headphone “y” splitter cable. Cut off both ends of the “y” and strip back the insulation. Connect the two “y” cable center conductors to the (+) terminals of two amplifier channels, and the cable’s shield to the (-) terminals. Connect a jumper from the (-) terminals to the ground terminal.