I recently acquired the XTC open with the biowool driver and the XTC closed with the beryllium driver, except the XTC closed is a slightly different version with larger cups that I believe is being called the XTC Squared. I will have to ask John again what all exactly is different about it compared to the standard version, but it definitely has larger cups. I believe the end result is even better resolution and detail. I must say I have been blown away by both of these headphones. I haven't done a lot of A/B comparisons against other headphones yet, because mostly I've just been enjoying them and getting to know their sound signatures before I start making any bold proclamations. I'm pretty certain however that they do compare very favorably with some of the best headphones I've heard. The only open backs I've owned are the 6XX, Arya, and Susvara, so I don't have a ton of experience with open back headphones, and it's kind of hard to compare dynamic headphones to planar magnetic ones, since they sound so different. My XTC open with the biowool driver doesn't have quite the clarity or resolution of a Susvara of course, but it can still be just as enjoyable, depending on what you're looking for in a headphone.
Closed backs, on the other hand, I've had more experience with, and my XTC closed with the beryllium driver is right up there with the best I've heard. A few of the others I have on hand to compare against include the modhouse argon mk3, ebony foxtex, audeze lcd-xc, sony mdr-z1r, sennheiser hd820, meze liric, kennerton gjallarhorn gh50 modded by John, and the kennerton rognir planar. I will try to do some A/B testing this weekend so I can add some thoughts on those specific comparisions.