You could get a KRK 10" sub to match your monitors. You would take the cables from your Audio 2 that are going to your monitors and connect them to the ins on the sub. Then take new cables from the outs on the sub and run them to your monitors.
oh my gad that krk subwoofer is freaking amazing, but money wise not quite an option.
any other recommendations? and chris thanks for the edit i really don’t have much experience in terms of sound and wires and cables and all that jazz but i’m getting into it slowly