I'm going to give the low-cycle FDW EQ a try to see how it affects my system. Sounds like it worked well for you.
I hope it works out for you! I'm not sure it will work for everyone. It may just work well for me because of the exceptionally even coverage from my SEOS horn-based 2-way speakers. My thinking was to try to capture the anechoic response and make that flat. Along those lines, the FDW window length must be long enough to allow for crossover delay (which usually peaks at the actual XO, in terms of FDW cycle count), but not so long as to allow any close room reflections to interfere.
The bass may still need to be treated differently in some cases. At some point, you go low enough in frequency that reflections and/or diffraction effects arrive too soon to clearly distinguish the first arrival from the effect of the diffraction/reflections on it. My inclination is to then treat the response combined with these early effects as being the first arrival. However, this may result in insufficient bass. My thinking is that pro monitors are usually designed to be anechoically flat, and they often get used in the mixing room either as-is or with only narrower EQ corrections for room modes. The implication of this is that the "first arrival" (including any reflections that can't be isolated) should be a bit hotter in the lower bass (say below 100 Hz) because the baffle step compensation built into the speakers effectively overcompensates once you go low enough that the strongest early reflections are constructive.
Of course, part of the reason I'm making this argument is that I am running my subs a bit hot with respect to the 2.2 cycle FDW, and I think it sounds better (as in closer to reference) that way. Another possibility is that my MBMs sound better hot because they are so close to the MLP and effectively higher in directivity. In other words, they don't put as much energy into the room for the same SPL as the speakers would if they were playing that bass instead.
The only real change I've made since my post above is to bump the subs up a bit higher. I pushed the MBMs (frequencies between 50-110 Hz) up +0.5 dB, making them about +2.5 dB hot at the MLP when using the 2.2 cycle FDW. Any more than that, and the sound is too slow and lacks punch. The bigger change was that I bumped up the deep bass subs (frequencies under 50 Hz) by +2 dB, making them about +2 dB hot at the MLP in terms of the 2.2 cycle FDW. I tried going up a full +2.5 dB to match the MBMs, but that extra +0.5 dB was enough to totally kill both transient detail and tactile sensation and make the bass actually sound wimpier. (Note to readers: you read that right; excessive deep bass can kill your tactile response by masking the higher frequencies that contribute most of the sensation.)
After the changes, the timbre of the lowest notes is a lot more consistent with the higher notes on the same instrument, and kick drums are very satisfying. The adjustments also tamed excess brightness, particularly in the top octave, on a lot of tracks. (Deep bass seems to mask top octave content to quite an extent.) Indeed, after these adjustments to the bass, I am finding the ATSC treble roll-off to be very satisfactory with a majority of recently released musical content. These changes did not fix excess brightness for all content. I'm fairly certain that there will always be content (particularly produced and/or mastered from the '80s and earlier) that sounds better with slight a downward slope (about -1dBish) toward the high end. I have also noticed that quite a few releases from the mid-to-late '80s still seem too hot in the top octave. This could be because monitors of that time were rolled off too much to try to compensate for the bad precedent set by earlier monitors. Another possible explanation is that the excess is due to top-end roll-off in the DACs used in the monitoring signal chains of that time. These older DACs may have rolled off at the very top because of the difficulty of creating an analog filter that cuts everything above 22.05 kHz (Nyquist frequency for CD audio) without harming response below 20 kHz. Most of these older DACs probably had some roll-off at the very top as this was arguably a less bad compromise than the other options. Oversampling DACs which have been in widespread use since the '90s address this problem by resampling to a much higher rate before conversion, giving the post-conversion analog filter much more room (in frequency space) to work with.
All of the above applies to music, and apart from the issues I've noted, music seems to be remarkably consistent as far as tonal balance is concerned. In the absence of specific standards, precedence rules, and engineers strive to follow the precedents established by the huge body of existing recordings. Movies are a different story, unfortunately. Despite there being standardized calibration methods, movie sound tracks seem to vary a lot more in terms of tonal balance. It would seem that standardization only actually helps if those standards are psycho-acoustically relevant. And unfortunately the methods prescribed by the movie industry are a long off, even with recommended adjustments to playback level and target curve made for room size. Indeed, standardization of this kind may have actually been counter-productive to the extent that such standards have been followed blindly. The X curve in particular has probably contributed to a lot of movie soundtracks having too much upper mid and treble compared to typical music. This is one area that near-field "for the home" mixes are said to improve on, but of course, the practices involved here are mostly non-standard and inconsistent. (I say *mostly* because apparently some effort has been made, by Brian Vessa of Sony and others, to standardize home mixes, but the information on how this is done does not appear to be available to the public nor is it clear that these standards are actually being widely adopted.)
I haven't watched a whole lot of movies since I found my reference response for music, so I don't have an especially wide sample of content with which to formulate an opinion. With that said, movies do seems to generally run brighter than music. Even the "made for the home" mixes seem like they might sound a little better with the -1 dB slope I mention above for older music. My guess is that some of the studios monitor the tracks this way on purpose under the assumption that it better reflects what speakers at home are doing. I would guess that they are correct that most home speakers have the slight downward slope because this does sound better with older recordings and because excessive brightness is usually more offensive to the listener than the opposite. However, I'd argue that they should still monitor home mixes flat (albeit with a roll-off at the very top similar to ATSC) to sound best on today's state-of-the-art monitors and to be consistent with modern music releases.
To deal with inconsistent tonal balance in movie releases, I plan to eventually implement a few controls into my DSP system to allow fine adjustments to the upper mid, treble, and top-end response. While such controls will be nice to have for older music, it appears they will be critical for getting the best sound with a variety of movie soundtracks. It's important to keep in mind that even 0.5 dB too much treble can add a few dB of loudness to a track, so if (room appropriate) reference level is too loud, there's a good chance it's just because the balance is too bright.
And just to reiterate, the sound I'm hearing on my system now (for music especially) is just stunning. The bass is strong and tactile but remains in balance with the mids or treble. Cymbals and hats are incredibly clean, detailed, and realistic (subject to the quality of the recording, of course), yet there's no harshness or fatigue even at high listening levels. I get decent tactile bass slam even from loudness war music, but of course, dynamic music is way better. I have some good dynamic live music recordings where the sensations from the kick drum are very strong, like something blowing up inside my chest.