If your sub and speakers are out of phase near the crossover point because of placement or room issues, you aren't going to hear that crisp pluck or snappy snare drum. Also for that purpose, the brickwall filter is actually beneficial since the sub stops contributing any output to the upper harmonics which are more deftly handled by the speakers. If you are using a mono EP600 with M80s with music, I would strongly suggest not going higher than a 60hz crossover and ideally with a 40hz crossover since the M80s can easily handle the stereo bass duties down to 30hz. I'm assuming also that you have a powerful enough amp to drive the 80s as they will need more power with the lower crossovers. Lowering the crossover should reduce artifacts which are likely caused by several of the above noted problems.
Very informative post jakeman. What are your thoughts of raising the sub xo if you have dual subs in the front corners? I would think it would be okay because, although the subs are now outputting a frequency that could be localized, since they are in the corners equidistant, it would still keep the soundstage balanced. Also, you would not be relying on a weaker amp to drive the low frequencies.
I tried xo's from 40-150, and the best response I got with 2 EP500's in the front corners was 90Hz in the AVR, subs in bypass.
