Typically, a decent studio subwoofer (such as the M-Audio SBX10 shown below) offers you all the connectivity you need. Instead of connecting your Avid MBox to your Eris 3.5, you connect it to the L and R INPUTS of the sub. You then connect the L and R OUTPUTS of the sub to the L and R INPUTS of your left Eris 3.5 speaker. You can then control the crossover frequency between the sub and the speakers. However, I do understand that the Emotiva Ultra does not provide this type of connectivity.