Well, I guess I'm more ignorant of this whole subwoofer thing than I thought.
I've always felt the beneficial use of a sub was more along the lines of an 'addition' to - or 'reinforcement' of - bass frequencies that were either missing, or had insufficient strength, from the main speakers.
I didn't realize that one should attempt to completely remove all bass frequencies from the mains through filtering when using a sub. Main speakers are usually designed to handle nearly the full audio spectrum. I never contemplated any reason to deprive them of doing just that. But it appears I'm mistaken.
I've not attempted to remove all high frequencies from getting to a sub any more than I've attempted to to remove low frequencies from getting to a set of full-range mains. After all, I had assumed the sub manufacturer wouldn't want (or allow) his speakers to attempt to reproduce violin notes or cymbal brushes. I was under the impression that this filtering out of high frequencies was done by the sub itself. Furthermore, I never even considered removing an entire frequency spectrum from speakers (mains) that were designed to reproduce it. To me, that would seem counterproductive.
Also, since very low frequencies - coming from the sub(s) or the mains - are relatively non-directional in nature (am I correct in this assumption?), I see no harm in allowing the mains to reproduce whatever they're capable of.
Unless, of course, the bass from the mains is so distorted as to destroy the musical content . . . in which case I would think a better course of action, rather than filtering out the lows, would be to replace the mains.
I'm always willing to learn, so please educate me, if you will. I don't see where allowing the mains to do what they were designed to do would be counterproductive.
Jim