On a typical AV receiver, setting mains=large, sub=on, and crossover=80Hz will (for Dolby 5.1 material) send ONLY the LFE track to the sub, whereas the mains get full range material. IOW there's no bass redirection from mains to sub -- the crossover is typically not used (for mains->sub redirection).

Likewise setting mains=small, sub=on will send LFE + redirected bass below crossover to the sub. Again this assumes discrete 5.1 material, say Dolby Digital 5.1.

But what about stereo material? If mains=large, sub=on, will any bass redirection happen from mains to sub?