Using a high level input (speaker cable) should be a last resort. It is a compromised implementation of a subwoofer. I'm not saying don't do it, but you are leaving a lot of performance on the table.
Subwoofers are most effectively deployed in systems which have discrete channel processing. Ideally the knobs on the sub should not be used for anything other than the onboard amp gain setting. Low pass should be set to max or bypass. Phase should be left at 0 degrees.
There is a specific configuration that is optimal for subwoofer performance. It is unique to each setup and requires calibration using a transfer function tool to align phase properly. Not unlike using a colorimeter to calibrate a display. The results are quantifiable.
The difference between 50-80hz crossover requires a change in delay offset to properly re-align. Toggling between 50-80 without re-alignment is readily perceptible once you first properly align for either frequency.
When the sub is properly aligned with the mains you can get away with a slightly higher sub trim level without starting to bloat the presentation. You can load the room harder before problems start. This allows you to protect your mains from over excursion without loss of output or fidelity at higher SPL.
Hope this helps you chase the last few % TDIPablo