The two sub modes probably deal with when setting the speakers to large or small. I don't own a sherwood put I guess the sub plus is for when you set your front speakers to large and sub norm is for when you set your front speakers to small...so try setting all your speakers to small in the receiver with a crossover of 80 using sub normal mode.

I’m armed and I’m drinking. You don’t want to listen to advice from me, amigo.

-Max Payne