Do No Harm ... well the least harm anyway

@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 -?

@immathewj

Yes, I run the subs to the high level inputs off a speaker output from an amp.  In my case, I reverse the polarity at the sub input, but some subs off it via a switch.  

My filter is 80hz, 24db slope. I can quickly bypass it by just joining the two XLR connectors and turn off the sub. I've done the comparison several times and find the imaging and balance improved with the filter and sub. And of course I can turn off the sub and the mains still sound great, if thin. So I'm comfortable that the filter does no harm.

Integrating the sub required adjusting the frequency and slope of the LPF and adjusting the phase of the sub. I played with PEQ but ended up not using it. If you are using an external amp, I would suggest some kind of DSP as well.