Using my m9XX with my HD800, some thoughts on the pairing:
1) un-EQ'd it's a bright pairing. Not harsh per se, but definitely bright and lacking bass extension. This, however, is simply how the HD800 sounds. The flip side is that it's a supremely detailed, spacious, dynamic, fast, transparent pairing as well. With Filter 2 on and crossfeed off, the soundstage is spacious, but a bit unnatural and disconnected. Crossfeed reconnects the soundstage. With the Grace's crosfeed, most headphones can get a little bit claustrophobic, sounding less headphone-y, but also less spacious. With the HD800 however, it takes the HD800s gigantic, but a bit artificial sounding soundstage and makes it startlingly realistic. I've heard bigger soundstages, but I've never heard a more realistic soundstage than this pairing (well, except what I'm about to talk about next). While I like this sound (HD800 + m9XX + Grace crossfeed, no other adjustments), it's clearly a thin sound. A detailed, dynamic sound, but yes, thin. It works for a lot of recordings extremely well. But on some recordings, especially rock recordings, it leaves a bit to be desired in the visceral impact of the music. It's like the soul got sucked out. The soundstage is also a smidgen less large than I'd ideally like, but is also perfectly acceptable.
2) Enter the above with the Sonarworks plug in. For those who don't know, this plug in simply takes measurements of the HD800, and inverses them to match your target frequency response curve. With the sonarworks plug in, we get all the benefits of the above, plus a startlingly lifelike timbre. The bass is full when it's supposed to be, the highs are delicate yet never harsh. The midrange is accurate, flipping from lush to urgent as the music does so.
One thing to note with the Sonarworks plug in is that it requires you to run your amp at a higher power level, as it limits the input signal so that it doesn't move in to clipping. With the curve I use, I usually have to run the m9XX about 8 dB higher than I do when using the HD800 unEQ'd. So, if you're a very loud listener, you might start to get into the 90s, where the Grace is slightly less clean, or if you listen crazy loud you may actually run out of power. I personally haven't gone over 85 with the sonarworks plug in, but it is something to be aware of for people listening very loudly.
3) The final piece de resistance for me was the Goodhertz can opener crossfeed DSP plug in combined with Goodhertz's free mid-side matrix plug in. I like the Grace's crossfeed circuit a lot, but it's non-adjustable and they clearly erred on the side of being conservative with it. Which was a good choice given the variety of music and headphones that would be running out of the Grace. However, to me, it always ended up sounding a tad bit like it wasn't enough, but also that it was too much. ie the crossfeed circuit narrowed the soundstage too much, while also not completely connecting it. When I say "connected" about soundstage I mean the ability to turn the natural headphone soundstage of "hard right, middle, hard left" into a continuous, smooth soundstage with no "drop outs". With the Grace's crossfeed there was still a tiny bit of drop out at about 2 oclock and 10 oclock. With the Goodhertz crossfeed system I was able to get it so that the soundstage was completely connected seamlessly. If I kept my head still and closed my eyes, envisioning musicians on a stage, I would get that magical "things slip away and you think you're there" experience faster and more dramatically than any other headphone setup I've experienced. However, the soundstage was a bit closed in even though it was realistically connected. That's where the mid-side plug-in came in to play. Giving the sides a 3.5dB boost I was able to "stretch" out the soundstage, without making it disconnected again. I was able to experience a full, connected soundstage that was also wide, deep and spacious.
Putting this all together is the most wow experience I've ever had with headphones. I got the transparency and dynamicism that only a headphone like the HD800, Abyss or SR009 can deliver. However, I get a much more balanced sound than any of those headphones come with stock. Timbre sounds absolutely spot on. I've heard Joshua Bell's violin in real life from 20 feet away and this sounded exactly like that. Spacial cues are interpreted and reassembled better than I've ever heard a headphone do. The sound somehow manages to be simultaneously analytical and musical. If I want to peer into the detail I can just try to focus on it, if I want to sit back and relax and enjoy it, that's totally possible too, it doesn't smash you over the head with harsh detail saying "LOOK AT ME" the detail is there, if you want it, but it can blend if that's what you want as well. It's up to you and how you want to focus on your attention. You can pay attention to a given instrument, hear the pluck of the guitarist snapping with nails and notice the difference between a flat pick. Or you can "step back" and take in the totality of the band. It's the closest sound I've ever heard to the pair of $250,000 Focal speakers I get to listen to every so often. Obviously not 100% there (headphones will never have the full body impact of speakers and with headphones as soon as you turn your head, the illusion falls a bit, as your brain expects the sound to change and it doesn't). But dang startlingly close for around $1500 total (HD800 $850, Sonarworks $79, m9XX $499, canopener $79)
So, that was a lot of words, and the m9XX barely got mentioned. However, I think my point is that this device can absolutely scale with the HD800, as part of a whole system. It's not going to fix the HD800's frequency response issues on its own, it simply reflects what the HD800 does naturally. However, with the right system, you can take advantage of the HD800's strengths, while nearly completely fixing its weaknesses. I'll likely end up buying an amp specifically for the HD800 (probably a torpedo from beezar audio) to pull out that last .01% of performance the HD800 can deliver. But make no mistake, the m9XX is a more than capable performer with the HD800, as long as you aren't expecting it to, by itself, tame the HD800's 6kHz peak and add 8 dB of bass boost. There are other tools better suited for those issues. However, the m9XX splendidly does its portion of the job.