This commercially available product claims to do what you need:
https://russound.com/products/audio-systems/multi-room-controllers/expansion-devices/adp-1-2-speaker-to-line-level-adapter#product_images
it is basically a voltage divider. It has such high values of resistance that it will not change the load seen by your amp. You will then have to use the Low Pass control on the sub to limit the frequencies to the sub.
And the volume control of course.
Personally I would just build my own, but that's just me.