Do No Harm ... well the least harm anyway

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.

@mashif

Good to know. What is the slope and lowpass frequency on your sub? Also, did you lose any of the "magic" with the Marchand? Specifically fine details, depth and imaging.

@koestner

Ha - Thanks for the reminder since I had forgotten that I listed the filter - no replies yet. 

The Marchand passive filter (XM446XLR-A) worked as advertised but it simply didn't lead to an audible improvement in the sound of my system.  It was quiet and didn't make anything sound worse, so what I learned is that the high pass filter is not needed in my system for my speakers, amps, and room.  The reasons are probably because of the power handling ability of my main speakers, the relatively high power provided by my amplifiers, and the control of the bass offered by the two subs that do not excite room resonances.  My main speakers (dual 9-inch woofers/side) roll off fairly low at around 40Hz and so rolling in my subs at about 45Hz seems to work well and it sounds good without the high pass filter.

your speakers have great low end.  high pass is a waste of them imho.  

if you have time and money to burn then use mini DSP between amp and preamp and dont bother unless you are good at RTA measuring.  

@knotScott , can you expound upon this?

Every room and situation is different, but I find my subs sounds best 180 out of phase,

How do you achieve this?  Are you running your subs via speaker wire input from amp and one sub is hooked up + to +/- to -, and the other sub is hooked up + to -?