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

I don’t use subs at home, but I did when I did pro-sound. Once you take the frequencies from 100 or 80 out of the mains, it allows them to play much louder without strain and the load on your amplifier is considerably diminished as well. But the tonal balance will be changed somewhat and that’s why it can be difficult to integrate a sub into a high-end system unless you know what you’re doing and what you’re hoping to achieve sonically. Some believe a sub should not be noticeable except where very low bass is present and some prefer it to cut in a bit higher to make the low end more cushy. That’s up to you. The problem is that every room is different and your results will vary depending on how the sub reacts with the room. Just as an aside, the old Janis sub, considered the best in its day, had a fixed crossover at 100 hz.  For what it's worth. 

Virtually every review I have read remarks on how the mids and upper bass from the main speakers are improved when you add a subwoofer.  I have found this to be the case as well in the past when taking the woofers and crossover out of my four piece system and running the mains full range by themselves.  You might want to play with different crossover frequencies (and, if possible, different crossover slopes) to see which might work best with your particular speakers and in your room--remember that there will probably be some overlap of frequencies, depending on the crossover slope you use, and there might be room nodes that are in play (I have a nasty one now in my room at about 50 Hz) that might be activated in the frequency range where the subs cross over with the main speakers... 

look at the new parasound halo integrated amp
it comes with built in crossover and good internal amp

I am going to use the halo amp to tri-amp my system 1digital.

i have the maggie 3.7 speakers driven by Oddesey mono amps 
coming off the halo crossover at 80 hz 

I have a pair of VMPS larger subs with alumn drivers , coming off the low frequencie output of Halo at 80hz , run by two crown mono amps (900 watts each) 

the halo integrated amp will be powering a mini maggie system 
with 2 bass panels and a set of mini maggie satellites that I will use
to fill any holes in imaging and  just get a much more full sound.
They integrate well with the maggie 3.7 , much bigger sound. 
the maggie bass panels will integrate the vmps subs and the maggie 3.7
the bass panels will improve the Maggie bass and the VMPS subs will handle all the low frequencies below 80 hz where you dont notice them ,

If you low pass your mains at 80 hz, this could be a less expensive alternative to see where your at. 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.

http://www.parts-express.com/harrison-labs-fmod-inline-crossover-pair-100-hz-high-pass-rca--266-274

I have been looking at those fmod filters as well. Has anyone tried them? Are they fairly transparent?