Horses for courses, I suppose. To me the 660S is just a better 650. Not 100% sure what ‘exposed’ means but I assume it’s not good, and I’ve never heard the iBasso so no comment on those (and assuming the 6XX is the same as the 650, because I’ve never heard a pair of 6XX but I’ve owned and loved and modified 650 (and 580 and 600 for years)), to my ear and in my system, the 660S is tighter, faster, better detailed, better balanced, more lively, more extended in the treble, better defined in the bass, better image, more space and air and more transparent than the 650. To me it’s like all the good neutral stuff from the 600, with the warmth of the 650 and the life of the 700 - still not perfect, of course, but we’re inching our way closer. The 650 is a lovely, musical headphone, and has been one of my go to’s for a long time, but I can’t think of any way in which it clearly betters the 660S - quite the contrary, IMO, listening to them back to back leaves me thinking the 660S is the clearly more advanced product (as it should be, 18 years later). Of course, different ears, different amps, different sources, different moods, who knows - I can change my own mind about headphones 10 times a day - that’s why it’s a hobby. But right now I’m loving almost everything about the 660S, and I’ve got a pair of 8XX in house to compare them to back to back (the 660S are my favorite from the midrange on down, with better tonal saturation, punchier although not as well defined bass, more musical muscle and simply more meat on the bones; the 8XX are the winners on treble extension, air, space imaging, build quality and comfort - no surprises, both great but the the 660S is more to my taste. It’s biggest problem is they didn’t put it in a fancier package, call it a 700S and charge an extra $300 list for it. Just my $,02, YMMV.).
And, hey, the Blue Dragons you recommended haven’t even arrived yet! Lol!!