For about a week I’ve been down with a serious bout of a rare form of migraine headache known as a "cluster" headache (I started getting them forty years ago). Imagine getting a bolt of lightning to your brain, and waiting for the next one to hit. Sometimes it’s in a matter of seconds, other times minutes. My clusters usually last about three days (most often starting just above one ear, making it’s way across my head to the other ear), this time the longest ever. The prescription drug I take usually helps, but this time didn’t. People have been known to commit suicide to end the pain of their clusters.
Anyway, catching up with this thread has been for the most part delightful; lots of great comments from knowledgeable, informed audiophiles. As for the others, oh well.
As others have already said, upgrading the crossover in your loudspeakers can be done without resorting to buying a kit from GR Research. And if you like a deep hole in the frequency response they may produce (due to two drivers being out-of-phase at the x/o point, or as in the original version of many of the Klipsch models, the woofer and tweeter not even reaching each other until their respective outputs have dropped way below the mean response of the speaker), that is of course your right. But to call that defect a "voicing choice" is just silly. What it really is, is poor engineering. Klipsch corrected the poor x/o filters in the Mk.2 versions of some models. That is not a matter of a natural evolution (or tighter parts values control), it is correcting a mistake. It was obviously done in response to Danny Richie’s evaluations.
There is no doubt that Magnepan’s introduction of the X Series versions of their models was also made in response to Danny Ruchie’s evaluation of a few models. I mean, Magnepan has been making all their models with the same crossovers since 1970, the X Series upgrade appearing only after Danny’s videos aired.
Here’s something to consider: I don’t know the prices, but it could be that the GR Research upgrade kit for the, say, Magnepan 3.7i, might be about the same price as the cost to get the X Series version instead of the standard 3.7i. But here’s the deal: Magnepan uses all better parts in the X crossover, but the x/o filter characteristics are no different from those in the standard model. It in no way "corrects" the problems Danny Richie found in his examination of the 3.7i.
What problems, you ask? The same problems John Atkinson found in the last Stereophile review of a Magnepan he did, decades ago. Magnepan has never sent another speaker to the mag, Wendell Diller saying it was because Magnepan’s can’t be measured like other speakers. Both Atkinson and Richie found the drivers played "over each other", a result of the shallow x/o filters and the chosen crossover frequencies. That creates serious problems of comb filtering, a phenomenon known to speaker designers for many, many years. It’s a testament to the quality of the Magnepan planar-magnetic drivers that the speakers sound as good as they do in spite of the flaws in their crossovers!
The beauty of the old Maggies (like the 3.6) is their series crossovers. With a good active crossover (like the First Watt B4 I mentioned earlier), you can create your own filters. I don’t remember what filter characteristics Danny came up with for the 3.7, but with a x/o like the B4 you can try 4th-order low- and hi-pass filters at 400 Hz. If you don’t like it, try something else.
Or, you can just buy a pair of Eminent Technology LFT-8c’s. A single push/pull planar-magnetic driver (magnets on both sides of the Mylar diaphragm) for 180Hz up to 10kHz, a ribbon tweeter for 10kHz up, and a dipole woofer for 180Hz down. Wendell Diller has been insisting forever that a monopole woofer "does not work" with a dipole loudspeaker, and Magnepan has been working on a dipole woofer system for a number of years. Why so long? No need to wait any longer, Eminent Technology already has one.
Or even better, add to your dipole loudspeakers a pair of the unique OB/Dipole Servo-Feedback Woofer that Brian Ding of Rythmik Audio and Danny Richie collaborated on. Open Baffle (2 or 3 12" woofers in an open baffle frame), dipole output, and servo-feedback control of the woofers. It will play up to 300Hz, unique for a sub. It comes with a plate amp that contains a dipole cancellation compensation circuit, and all the controls you want and need, and in the analogue domain.