So, I am about to set up a new pair of monitor speakers and fill in the lower frequencies with two subs. The monitors are Aerial LR5s and the subs will be two Aerial SW12s with internal 400 wpc amps. The LR5s weigh 105 pounds each and play flat to just a bit above 40Hz (-6dB at 35Hz). The Aerial subs are fast, powerful and musical and should integrate well with the LR5s. My pre and power amps are solid state, fully balanced and the power amps provide 300/600 wpc into 8/4 ohms. I will be running separate ICs from the preamp to my power amps and a second set from the preamp to the input on the subs.
My first question is whether I would benefit from using a high pass filter between the preamp and amplifiers to limit the low frequencies going to the LR5s so the amps are not trying to drive them full range.
I didn't find a DIY design that I was comfortable with but I did run across several posts by people using a Marchand passive high pass filter. The Marchand XM446XLR-A is fully balanced and has a high pass corner frequency of 80Hz (i.e., the frequency that is down by 3dB). The Marchand has a standard slope of 24dB/octave but can be made for other slopes.
My second question is whether anyone out there has experience with the Marchand passive high pass filter and is it sufficiently transparent or should I expect to hear unwanted artifacts with the filter in my system?
Finally, is there a higher quality alternative or should I simply play with the low pass on the sub and forget about filtering the signal to the main speakers?
http://www.marchandelec.com/xm46.html 
