Subwoofer Connection: Speaker Level Amp to Line Level on Sub?

You should not do that. Speaker level voltage can be much higher than line-level audio voltage, and the speaker outputs are expecting to see an impedance load in the 10 ohm range, while the line-level input is probably in the 10,000 ohm range.

from speaker level on the amp to line in on the sub using an adapter?

It should be fine if you connect the sub line level rca inputs to your amp speaker blinding posts via a LOC (line output converter).

https://images.crutchfieldonline.com/ImageHandler/trim/1736/1129/products/2008/543/x543ADP12-f.jpeg

Thanks for the input.  There are better solutions for this situation than fooling around with adapters.  Gonna drop back and punt.