I'm looking for feedback about situation I have with my HT set-up but posted it here as it applies to any ported speaker in general.

What effect is there when electronically crossing a speaker above the tuning frequency of it's port? I've always used sealed speakers in the past, and when listening in two channel stereo I don't bother with a subwoofer. For HT use, I get pretty muddy response if I combine the ported mains playing full range with a sub crossed at pretty much anything, even as low as 40hz. I've tried crossing the mains at the same frequency as the sub, but seem to get a midbass gap. I know that a port's contribution to overall sound increases as you approach it's tuning frequency. Does this mean that the frequencies leading down to that tuning frequency are also affected by the x-over point? Could this be causing my midbass gap when using same x-over point for sub and ported mains?