Find a sub with speaker-level inputs and outputs.
- If the sub has inputs only, run a pair of wires from the amp to the inputs and also a pair of wires from the amp to the L+R speakers.
- If it has inputs and outputs, run a pair of wires from the amp to the inputs, and then connect from the outputs to the L+R speakers.
Use the LPF on the sub to blend it with the mains.