1. Don't connect the chassis
2. Connect the red/yellow wire from REL to both connectors circled in blue.
In the opinion of our engineer, connecting from speaker output is not a good idea for sound quality. In the old days, the understanding was to have similar sound characteristic for the subwoofer and amp. And this method is used for preamp that has only one output. You don't need to do that anymore.
Anyway, what matter is what you hear, so let us know how the two different ways compare.