High level simply reads the speaker signals, and therefore requires your speakers receive a full range signal. In order for that to happen on an AVR, bass management has to be disabled by putting it into pure audio (or whatever its called on your amp) mode. You then use the crossover on the sub to properly integrate it with your speakers natural low end roll off.
Using the normal LFE connection, you simply set the crossover to max and let the AVR do the rest, ensuring your AVR is in a proper x.1 mode.