Subwoofers and Crossovers.

I agree with Yogi.... try running the speakers full range with the subs low pass set low, around 60 to start .   Often times it sounds better....  A crossover that let's you experiment with the high pass is also advantageous.  80Hz is too high and the main speaker looses some "meat on the bones"   Too high of a low pass and the sub becomes directional.  You can hear where it is in the room.   

I use a Velodyne sms 1 for sub eq and to roll off the sub sharp at 50 hz.  You hardly hear it because my main speakers are full range but it integrates well. The JL crossover will let you do that....  

The Spendor D7.2 are different animals with respect to subwoofer integration.

Their quasi transmission line bottom firing port extends bass to 25Hz.  When attempting to add 3 sets of subs to them without high pass filtering the main speakers, the sound was horrible at worst- cancellations, hard sounding bass, lack of seamless integration, affected the sound overall in a bad way.  

Adding in a high pass filter and you lose that quick, tight quality that these speakers have.  

No subs for me and the Spendor D7.2 and I am more than fine with that.  

@khiak Those Funk Audio subs look awesome and am sure they perform well, but they are not for me. Not only are they beyond my budget but they are also really big.

@asvjerry If only it was so simple. I realize digital crossovers are really good, but I am reluctant to pass the analog output of my DAC through  an ADA process. I am starting to think I am being irrational here.

@dimbhaka, *L* It all seems irrational after a certain point imho, but....*shrug* ;)

I like to set the 'general environment' to flat response as much as practical, given that I'm running diy omnis' and dipoles in a 'highly compromised' space with one DSP equalizer, and tweak to taste with another.

"Compromise" as an artform....*L*

You might give it a go as an experiment with a minimum outlay.  I'm using the Behringer to pass the lower bass to a sub to keep the omnis' into the range they seem 'comfy' with, since lower bass freq. are 'omni' in nature anyway....

Works for me....if not for you, you can always curse me with whatever seems appropo.....*L*

I've already got the 'underwear into the crease' version, so try something different and surprise me... ;)

I use a MiniDSP SHD as an active crossover for my passive 18 inch subs. I also use the included DIRAC room correction that performs DSP on both the subs and the mains.  bought the SHD from George at Deer Creek Audio and also gladly paid him to set it up and modify the program when I make system changes. It's all done remotely using a laptop and Team Viewer software. I never wanted to run my entire signal through a $1300 box, but I don't hear the SHD in the circuit except for the nice effects of the room correction and the seamless sub/mains integration. It's not the solution I would have expected I would buy, but it works extremely well. The SHD also allows you to time correct for the subs being set on a different plane than the mains. The MiniDSP SHD has a huge feature set for the money.