Do No Harm ... well the least harm anyway

Well, there you have it. It seems I have about a snowball's chance in Hell of getting this idea to sound correct. I think I will let the Nefes play full range and let the subs give a helping hand under 50 Hz or so. My subs have all kinds of adjustments, so I can mess with them and not really worry about the critical integration of 2 different drivers at the 100Hz range. Thank you ALL!!

Adjusting your subs makes the most sense to me, but I do understand what you were hoping to achieve.  There's just never a free lunch, and the downside is likely to have more overall negative impact than positive, but in fairness that's an educated guess, not a fact.  

Every room and situation is different, but I find my subs sounds best 180 out of phase, low pass filter at the lowest setting, and gain set very low so that it's barely audible except on deep bass heavy passages.  YMMV, but good luck and enjoy. 

I am wondering about the same issue. Now it's very close to, as in high up, the meaning of life, on my list.

IME, the potential benefit of using a high pass filter relates to the quality of the low bass reproduced by the main speakers, the ability of the main speakers to handle power in the low frequencies, the ability of the amplifier to supply sufficient power to run the main speakers full out, and the response of the room and susceptibility to standing waves and excitation.

A fairly good quality Marchand passive, balanced, high pass filter set around 45Hz did not improve the sound of my system, which includes stand-mounted main speakers each having two 9-inch woofers in a sealed box (acoustic suspension) design that produce solid bass to about 40 Hz. In my case, the best sound was achieved by running the main speakers full-range, and rolling in my two subs at about 45Hz.

Using a high pass filter to roll off the OPs full-range low frequency driver could possibly offer sonic benefits, if using the filter keeps the woofer in a more linear range and reduces stress on the amplifier. An appropriate roll-off point might be just a bit higher than where the speaker’s frequency curve starts to drop off, but certainly much lower than 100Hz.

@mitch2

Your post reads like you are happy with the passive filter, but you say it did not improve the sound. Please elaborate if you prefer the Marchand or not, and is that why you are selling it on USAM? I'm not trying to be a smarta$$. Thanks.

I went with a Marchand balanced passive high pass at 80hz because of a room/speaker placement issue. With my speakers in the best sounding position overall, I had a big bump from 50-70hz that was resistant to acoustic treatments. By rolling off below 80 and filling that with a sub located near floor level, that bump disappeared without even using eq on the sub. 

Every situation is different. I spent months working on acoustic treatments that reduced the bump from 10 to 5db, which I think helped with integrating the two. The overall result is a great improvement though I expect I'll continue to experiment.