You dont "bridge" subs, you bridge amps. Bridging means to put 2 channels together to make 1 channel by using the + from CH1 and the - from CH2.

With the 300/2 bridge, it will make 300 watts at a 3-8OHM load. If you buy 2 8OHM speakers, and wire them in parallel, that will net you a 4OHM load.