Question: How do you connect these subwoofers to a receiver?
Answer: There are several methods to connect a passive sub.
The best option would be to get a dedicated, monoblock, subwoofer amplifer for each sub. You connect the LFE (sub out) on the receiver to the amplifier and connect the sub to the amp with speaker wire.
Another option would be if you have optional zone 2 speaker terminals on your receiver that are not in use or a bi-ampable receiver. Just connect the passive subs to the secondary speaker terminals directly on the receiver.
The next option would be to purchase a speaker selector. These devices will match the impedance across all the connected speakers so you don''t overload your receiver. Connect both your mains and the passive subs to the speaker select and make sure you have both sets selected.
The last option is to wire your speakers in series with the left and right front speakers. This involves going from the positive connector on the receiver to the positive on the sub, then the negative of the sub to the positive of the front speaker and the negative of the front speaker to the negative of the receiver. This is the least preferred method since, though it will lower the impedance of the chain to a safe level, it will also reduce the total output you are getting.