Got M&K MX-150 THX for Maggies... feelings?

Bob, I made it very clear that my remarks applied to panel speakers, which do not have increasing SPL in the bass and (for this reason) do not exhibit bass distortion since you can't have distortion where you have no response.

The bass response in (almost) all panel speakers (along with the SPL) tends to roll off smoothly starting between 40 and 80 Hz (depending on the speaker) at a slope similar to that found in a high pass crossover circuit anyway.

The object (again, with panels) is NOT to assign bass and mid-bass to two separate devices but rather to BLEND the outputs of the two devices as if they were part of a single enclosure/crossover system. Attenuating (sloping off) the sub starting at 10 to 20 Hz below where the panel initially begins ITS natural roll-off insures a nice flat response across the point where the two speakers' acoustic response overlaps, sort of a "natural" blending (ie in the air) of crossover slopes.

As for locating the sub, the dipole radiation pattern of panels produces sound waves between the front and back of the panel that are at approximately 90 degrees out of phase with the original signal (ie 0 degrees at the front, 180 degrees at the back, 90 degrees in between) so locating a single sub exactly between the speakers F/B/L/R, and adjusting it's phase (if you can) to 90 degrees, will make it virtually disappear and be indistinguishable from the panels.

Again, these comments apply to pure full-range dipole speakers (ie not hybrids with cone drivers as in many Martin Logan speakers.) Examples would be ML CLS's, Maggies, Quad 57's, etc. Soundlabs do qualify, but shouldn't need subs anyway ;--)

Some of your remarks DO apply to typical two or three way monitors or full-range box enclosure speakers; and I have certain other ideas about those applications. I'm quite familiar with M&K bass management system. I know Ken quite well (in fact I designed their logo back in 1970) but his recommendations apply (primarily) to multichannel systems with conventional speakers, LFE processing, etc. I still stand by my advice as to using subs with dipoles.
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Although most planar speakers aren't capable of the excursion necessary for low bass reproduction(and hence: roll off the bass)- It doesn't mean they aren't trying(not to mention the wasted wattage). The sound of your Maggies will benefit greatly if you use a GOOD high-pass filter(BEFORE your power amp). I've been actively bi-amping planars for the last 26 years, and have found they ALL sound cleaner/less strained/more dynamic with the bottom handled by another amp/sub system. The corner sub not only energizes the room and lays an excellent foundation for your music, but- the driver has much less work to do having the reinforcement of the room boundaries. The drawback is in the time domain. Three feet will add up to a lot of mS in delay. I'm using a TacT 2.2X which delays the mains to compensate(along with a multitude of other neat tricks).

Rodman99999, I did it that way too (for 14 years, not 26 years) until I realized it was the wrong approach ;--) I sure was happy to eliminate those external filters, delay compensators, etc., from the chain.

If your (very well designed, I assume) planar speakers sound strained, it's certainly not because you're runnuing them full range, it's because your amp is not right for the job. Planar speakers don't "strain" when they run out of bass response, the bass just fades away.

There's absolutely no reason to take ANY bass response away from the main speaker and send it somewhere else. If the main speakers produce crappy bass and it's not the amp's fault, then you should sell them and buy something better.

The purpose of a subwoofer is NOT to take over any bass duties from the main speaker but just to (subtly) fill in those regions where the main can't go.

If you'd like confirmation of this approach, you can speak with Steve Winey at Magnepan and/or Jim Power at Martin Logan. Steve will also tell you that bi-amping the old IV-A's, 3.6's and 20.1's is OK, but only if it's to put a tube amp on the tweeters -- not to cut off the bass response.
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Gentlemen,

I'm afraid that I'm becoming confused with the responses to my own question and now find myself even more puzzled than when I started. Considering that I have a great deal of knowledge to assimilate before I can take such technical observations into account, would I be folly to use my pre/pro's bass management to set a crossover at the -3db frequency of my Maggies and slowly turn up the M&K until I can match SPL at the point of crossover? Is this approach too pedestrian for the Magnepans?

I notice an MX-150 Mk II for sale here in the classifieds... I'm beginning to think about picking up a second! Would my money be better spent on the addition of an SMS-1 or some other EQ to smooth out what is an admittedly atrocious room than on a second sub at a good price? I hate the concept of limited resources with limitless wants... thank you, microeconomics, for my crippling indecision!