You are talking about the crossover in the AVR (same principles apply to a crossover in a speaker between a woofer and a tweeter)...
- the crossover contains two components, a high-pass filter and a low-pass filter; what gets through the high-pass filter goes to your speakers, what gets through the low-pass filter goes to your sub
- these filters will have a slope that define how the sound falls-off, typically 12db or 24db per octave (more knowledgeable folks will talk about 2nd order and 4th order slopes, way over my head)
- so if you st the crossover in your AVR to 80 Hz that means the two filters are set to 80Hz
- the point about the slopes is that the filters are not brick walls; so not only above 80Hz goes to the speakers, below it also goes, but the further below 80Hz the weaker it will be; if you had a 24db/octave slope then the signal at 40Hz would be reduced by 24db
- so at an 80Hz crossover, your sub will get pretty much everything below 40Hz, most between 40 and 80Hz, some between 80 and 120 and little above; your speakers will get pretty much everything above 120Hz, most between 80 and 120, some between 40 and 80, and pretty much nothing below
Clear as mud