First, I'm none too impressed by the reviews that look like they're from audioreview.com. Look, comparing them to cheap Sony's for most people means comparing them to V-crappyDJ phones. Beating those doesn't mean much. Second, we have no idea what else these people have actually heard. Yeah, the one guy said he'd heard all kinds of other stuff, and said only the HD280s came close. Well, lots of people will tell you that many cans from AKG and Senn sound
better than the HD280s. So I'm not sure how helpful these reviews are.
In addition, if you think the Senn HD595s, which are less than (though right around) $250, are "congested" and "muddy" then I'm not particularly moved by your opinion, either. (Though I could see that for, say, the HD477, no problem.) It almost sounds like you're trolling. Well, maybe not, your comments on what cheap Sonys sound like seem pretty reasonable.
And yes, Yamaha makes some good audio stuff, but they also make some crap, too. The speakers on this shelf system from them I have are singularly unimpressive (and have been replaced), and some of their computer speakers are equally horrific. So, saying "oh, it's Yamaha, it must be good" is not compelling. (Neither is saying that for Sennheiser--there are many Senn models which routinely get bashed around here, and not just the cheap ones, e.g. HD570. And I think most Head-Fi'ers would agree that most all the cheap Sonys are also awful.)
So, in answer to your question for what it would take? I'd have to hear them. Find a Head-Fi meet near you and go, and take them, and see what people think. (Try not to tell them exactly what it is they're listening to--maybe tell 'em they're some kind of Ultrasone prototype or something.
) That'd be much more convincing.
I will admit, though, that I'm curious a little, and if I'm ever in a shop that has these, I'd give them a listen.