Bob, first I'd just like to say that I feel it's probably a mistake to make generalizations about any speaker "type" be it horns, or boxes, or planar (magnetics or stats) etc. Some are very good at everything (they do) and others not. This is less true of planars but only because they're relatively harder to build, and so there's no profit in making crappy ones ;--) So saying that "a conventional box speaker produces more distortion in the bass region by either increasing SPL or decreasing frequency" is simply not true for the majority of decently designed and built box speakers operated normally (ie with a flat signal.) And I'm not just referring to megabuck products.
For the sake of discussion, I prefer to say dipoles because some "planars" are curved, and not all dipoles are planars -- some are infinite baffle cones. So, except for Soundlabs electrostats, Apogees (now coming back), and the big bass panel Maggies (old and new) I don't know of any panel dipoles (as opposed to cone dipoles) that go flat all the way down to 25 or 30Hz . And as most people eventually discover in the case of Maggies, you really have to play them loud to get them to perform "full range" -- but it IS possible, and they CAN be driven hard without distortion. My Tympani 1-D's had great bass (ie no need for a sub) but only at live levels and they absolutely required a powerful ss amp. If you wanted rich sound otherwise, you needed a sub or a "loudness" control.
Most planar dipoles (ie electrostats) just don't have the enormous panel size it would take to produce low bass -- even the big Soundlabs barely make it, but they do make it!
So with most of them, the bass output (SPL) just starts tapering off at (what would be for an ordinary box floorstander) a fairly high frequency -- beginning 80Hz for my CLS's and down to "you can't hear nuttin' at all" below 40!! BUT, you don't get any bass distortion, no matter how loud you play them.
So fact 1.) The bass response profile (generally) for dipole planars (including Maggies most of the time;--) mimics a typical high-pass rolloff curve that you'd find in a typical electronic crossover set to say 80 or 100 Hz at a slope of 6dB per octave, PLUS there's no phase shift. Knowing that, it sort of makes the use of an external high-pass filter or crossover redundant -- the speaker, BY ITS DESIGN has already rolled off the bass in the most natural way possible! The only way you could get it to distort, and ONLY at the frequencies above its 40Hz lower limit, would be if you tried to correct it's natural bass defficiency by trying to kick it back up with an equalizer.
So fact 2.) All you need to do is add a sub that has a high frequency rolloff curve that's the opposite of the panel's low frequency rolloff curve (in other words, where the SPL of both speakers falls to about half, at the frequency where the curves cross.)
As for why your Maggies sounded better with high pass filtering, with or without a sub -- I don't know, but something was not right -- it was certainly NOT due to an imperfect design. Perhaps it was the amp, or a failure in the crossover, or even a bad panel? I know the folks at Magnepan, and I can tell you they wouldn't put out a product that sounded better after the customer circumcised it.
Fact 3.) Placement: Dipole speakers radiate two waves, equal in strength and opposite in phase. And, there are tons of dipole sub designs, which would be the obvious choice to match with panels. UNFORTUNATELY, there don't seem to be any that are commercially available and/or have great transient response. I found that the new Martin Logan designs (the Depth and Descent) with their omnidirectional radiation pattern can be made to approximate a dipole sub, if placed right between the panels and set to a 90 degree phase angle. (Meaning the sub is only a LITTLE out of phase with the front OR back of the panel.) And, they are FAST! Martin Logan also has brought out the new Summit, which has its own self powered built-in sub each side. They sound great, but I'm not sure they solve the dipole phase-matching issue as well as my solution (one Depth in the center) or perhaps a real dipole sub would do.
As for subs-with-box speakers, that seems to be a two sub solution usually. Whether its by grouping sub towers with mid/high towers (Alon, Genesis, Pipedreams, etc) or with main full-range (?) speakers. The latter being something I just totally don't understand (why not just build decent bass into the main speaker to begin with.) I'm OK with the monitor/sub solution, but again, it's going to be a compromise no matter how it's done (ie one sub or two? and where do you place it/them?)
Basically, my "convictions" (I'm OK with that:--) are that for two channel, keep the system itself as pure and simple as possible, especially the preamp-to-amp-to-speaker part of the system. Think "minimalist". Don't add stuff to fix the signal when you should fix the room. Avoid the urge to bi-amp (unless the speaker comes with it's own bass amp of course.) If your room is smaller than 15.5 feet by 23 feet, accept that your speakers will be on the short wall and you will have to listen "nearfield." As for HT or multi-channel, I have no particular convictions because the object is to produce stunning effects whatever that may require.
