If you have not already done so, run the 2-conductor speaker cable from the location of the SDA-120 Amplifier, to this wall-space where the Subwoofer will go. Allow some extra cable length at the end for ease of connection. Strip about 3/8” of insulation off of the ends of the two wires in the speaker cable. But do not connect subwoofer at this time.
Cut two 1' pieces of batt insulation. Slide one piece into the wall cavity above and below the subwoofer location.