Cutting below 80 HZ on my mains, so subwoofers handle deep bass?

You really do not want to cross high and low at the same 80 hz frequency. A bit higher [high pass cutoff] is recommended rather than the same as the low pass filter cut.

So maybe cut 60 HZ and below for the mains, and set the subs at 80 HZ? Then there’s still a little play between the two. I see how not drawing such a hard line with the frequencies could help not create a hole in the audio.

You know, you could also just cut the subwoofer's low pass filters instead. :)

You know, you could also just cut the subwoofer’s low pass filters instead. :)

Setting the sub’s low pass to 30 HZ isn’t really worth it for 2 channel music. Maybe if I was configuring this for movies it would be a different story.

A lot depends on the speaker's crossover as well as the drivers.
I think overlapping the frequencies, as pts suggests, will result in less accurate reproduction.

Some things said here are inaccurate.

First, my experience is that the subwoofer settings are not "brick wall" filters and allow a good bit of transmission of frequencies above the setting, which may or may not be accurate anyway.

Second, neither are the crossovers in the speakers "brick wall" drop-offs at the rated frequencies, rather they continue to generate sound at lower frequencies on a modest rolloff of output as the frequency drops.

I have found that it is necessary to set the crossover frequency of my REL subs far below what is notionally indicated to avoid bass bloat. Going higher is a step in the wrong direction, IME.

Third, subwoofers integrate better with full-range speakers when they are felt and not heard. The 30hz and lower frequencies from the sub(s) generate sound pressure that loads the room, giving a perception of greater bass weight and also better soundstaging.

Dave