At about 40 hours of head time with the Lawton mods (I didn't use the foam or the piece that attaches to the wood cups, just the pads and the damping pieces that attach to the driver) and man, it's really blowing me away. While each individual aspect was a very subtle difference, they've really come together in a special way. The bass is so tight and impactful in the region right between sub bass and midbass (80Hz). Transparency is noticeably clearer, speed is faster and soundstage is larger. It doesn't have that warmth that the stock THX00 has, but is cleaner. It's obviously not as resolving or spacious as the HD800, but it's dang close in those regards the HE400i. I thought when I bought the HD800 it might convince me to sell the HE400i, and that hasn't been the case (they both have different strengths). However, these mods to the THX00 may do that. Not sure, I still have a lot of love for the HE400i, which still beats the THX00 in imaging accuracy and instrument separation by a bit, but the resolution and soundstage gaps have been closed. For another point of reference, I'd say that my THX00 now has about the same soundstage as the HD650. Macro-resolution is the same as the HD650, but slightly not quite on the HD650's level in micro-resolution (the HD650 has some of the best micro-resolution out there though, so no shame). I'd say that my modded THX00's bass is about as clean as the HD650, while being substantially above in level. Mids are liquidy without being as thick as the THX00.
downside is that it's no longer quite as forgiving of poorly recorded rock, but it isn't punishing like the HD800 is. Very similar to the HD650 and HE400i in this regard.
Here's how I'd lay out the mods individually and how they added up:
Ebony cups: small increase in sub bass, small increase in lower midrange and midbass levels, small increase in smoothness in treble, slightly less thick sound overall. Lage increase in aesthetic pleasure (haha)
Lawton Pads: decent increase in soundstage, small increase in sub bass, small decrease in midbass. decrease in efficiency (ie need a bit more oomph from the amp to match volume levels, small increase in resolution
Lawton damping (pieces that go on the driver only, didn't use the cup attachments or the foam): noticeable increase in resolution, less ringing, less THD, faster, small increase in soundstage
Again, any one of these mods, other than the pads, is a pretty subtle difference. But when you add them together, it makes for a noticeably different, and in my opinion superior headphone. There are some difference in signature, noticeably a shift from lower mid range and mid bass to lower treble and sub bass. But what has really sold me are the non-frequency response aspects. The increase in resolution, speed, transparency and soundstage have all made it much better to me. I admit that in some ways I do at times miss the thickness and forgivingness of the stock tuning on some rock recordings, overall, it's definitely a change I'd make again without thinking twice. I'm currently listening to Joshua Bell's BWV 1041 Allegro with the academy of st martin in the fields and this headphone can absolutely nail classical now. The room sounds more humid than with my HD800, but they both sound realistic. I kind of think of it like the HD800 is hearing the performance when your ears are really right, and the conditions are perfect. The modded THX00 is like hearing the performance after a couple glasses of wine. The old THX00 smeared things together too much for a subtle piece like this, like listening after 3 double whiskeys neat.
and again, this isn't to trash the stock THX00, I still think it's a great headphone. If you are mainly listening to rock, I probably wouldn't do these mods. The THX00 in its stock configuration is more or less perfect for your average rock recording. The modded version though is still very good at rock, but also can hit jazz, classical, bluegrass, electronic as well.
To put it in terms that are clear about how good I think this is, I would take this headphone over the Ether C and TH900, even if they cost the same.