Plug Rel Subwoofer into speaker posts rather than amp?

The only damage possible is in the audio via cheap crummy speaker cables. Probably negligible to a sub.

Here ya go!

If connecting 2 RELs, connect the black wire of each REL to a negative speaker terminal of the corresponding channel; twist together the red and yellow wires of each REL separately and connect each pair to the positive speaker terminal of the corresponding amplifier channel. In some instances, this will result in exceptionally high gain (output) from the RELs. If it seems simply too high in gain, please remove either the red or yellow wire from the twisted pair. This will reduce output by half and restore a natural dynamic.

Note: You connect your left and right main speakers to the same terminals on your receiver or amplifier as the REL high level connection. If more convenient you can also attach the high level cable to the terminals on your main left and right speakers without any loss of signal quality.

Thank you!