Do No Harm ... well the least harm anyway

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.

Hello koestner!  DBX makes a 2 way stereo/3 way mono electronic crossover (DBX223, be sure get the model with the kind of connectors/in & out plugs you prefer) and is transparent and reasonably priced. I use then in two of my 5 systems. They are a tweaker's delight, 24 db/octave and let you fiddle to your heart's delight. I agree that keeping the very low bass out of your main speakers is a good idea. I'm using Magnepan LRS and LRS+ in the systems with the DBX Xovers. My other systems use miniDSP electronic crossovers. Give each driver it's own amp. These days, good amps are not expensive. Happy listening.