To connect the B1500D-PRO (or B1800D-PRO) subwoofers to B212D speakers, follow these steps: 1) Start by plugging an XLR cable from one of the MAIN OUTPUTS on your mixer into the INPUT of the B1500D-PRO. 2) Next, use another cable from the OUTPUT of the B1500D-PRO to the B212D INPUT. 3) Repeat these steps from the second OUTPUT on the mixer to the next set of speakers.