Hello people. After following this thread I bought a pair of GM200s and have been using them for just over a week so posting my impressions. I have no proper experience with high-end headphones compared to the people on this forum so don't really have the frame of reference to speak about their relative sound quality etc., so this is just a short summary of how I've found them, from a noobie perspective, featuring some comparisons with another set of headphones.
Comfort/Isolation: As is well documented at this stage, these are amazingly comfortable. I had the logitech UE 6000s before which were really comfortable themselves and I was afraid I wouldn't be able to top, but these gm200s are even more so. The pads are so soft and cushiony. I was expecting them to clamp more onto my head but they don't, and they don't need to. It feels like everything's well sealed even without any pressure of them pushing in around the ears. I've worn them for hours at a time with no discomfort or overheating. I've also worn them on bus and train journeys and in the office, and been able to hear fine without the feeling that I'm annoying other people around me (at least no one has looked at me funny or complained).
Sound Quality/Bass: I listen to music that has a lot of bass and so chose these headphones due to the reports of the strength of their bass. As I said I haven't tried enough other headphones to know how these stack up and fit with the rest of the market, but my general impression is the music sounds really clear and detailed, warm, and quite spacious. It might be in my head since I have a new toy, but I feel like I'm hearing little details in songs I hadn't noticed before. I usually listen on a very low volume and at these levels the bass doesn't boom the way I had been imagining before buying them, but I can't say I'm disappointed; things just sound nice and warm and the bass does punch on songs that feature strong kicks and basslines. Overall it's a bit more balanced than I had expected, especially at really low volumes, but I really don't know how 'bassy' and unbalanced other headphones go and when sound quality starts to be properly compromised. When I want more umph and turn the volume up even a little bit the bass absolutely delivers and can really punch, and the bass sensation seems to grow and overtake other aspects of the music a bit more at these levels (this may well be technically untrue, but it's just the feeling I have). But they don't like absolutely blow your head off with the booming, especially at low volume, and everything still feels nicely balanced, so to that person worried about getting headaches I really don't think there's anything to be worried about. Not sure what else I can really say; I've tried listening to loads of genres of music on these and it all sounds great
My UE6000s after breaking actually randomly started working again so I was able to do a side by side comparison. To me it seems the GM200 have stronger and deeper bass, and generally more clarity. They also sound bigger and more spacious somehow, like there's more space around my head being filled with music,whereas the UE6000s now seem like the music is going directly into my ears in a more narrow tunnel and the music doesn't have this room to breath (sorry for the crappy descriptions here).
But yeah overall very happy with these so far. The logitechs folded into a carrycase that was really convenient, which these don't have, so the only thing close to a downside I've had so far is when I pack them in my bag I'm worried about breaking them, but that's really not a big deal. And this isn't due to build quality, my bag is just usually too full.
Thanks to BenF especially for all the info and answers on this thread. After stressing about what to buy and thinking I couldn't replace the UE6000s for under $100, I'm very happy with my purchase and prefer these new ones