I think we are talking about 2 different things. I think what you mean is the high level input on the sub.
I have a Denon AVR
If you had an older 2 ch receiver with no crossover then some look to the sub to actually provide a crossover (usually via low level input/output). With a Denon I'd not use the low pass/crossover on the sub, just the one in the Denon....
