So, when it comes to sound, the biggest flaws of the MSR7 were lacking bass (lacking quantity, quality was excellent) and an overabundance of peaky highs. I think they had a 5k and a 10k peak, but can't remember for sure. The MSR7b firstly added A LOT of bass, especially in the sub bass region. At stock, these give me more or less the perfect amount of bass. Enough to make them sound fun, but not enough to cause bloom or looseness. Plus, they retain the excellent timbre of the MSR7, so kick drums sound like kick drums and not electronically generated beats, for example. And then treble. The MSR7b sound much smoother. They're still somewhat on the bright side and SOMETIMES present some slight sibilance on SOME recordings, but nowhere near as much as the MSR7, they never seem to become obnoxious or fatiguing. That wasn't hard to fix, though. What's amazing is that I can't hear any drop in detail. I mean, I could slap a few filter materials on the MSR7's driver to quell the treble, but detail did suffer. Luckily, the MSR7 had way more than enough detail (insanely much for the price bracket). The b have more or less the same detail in their sound, but not at the cost of having to turn them down so that my ears don't bleed. When it comes to mids, I feel that they didn't change much from the originals. The MSR7 had excellent mids, and so do the MSR7b. Lastly, I think that the b may sound wider, but I can't really remember what soundstage was like on the stock MSR7, so my opinion is invalid on the matter and others may chime in. Imaging is excellent too, although not on par with my HD58x, but these things do imaging exceptionally well and aren't wide at all.
Now, build quality. There's no other way to put, I prefer the old ones. They just feel nicer on the hand. They are heavier, they feel more solid and more premium. BUT, they also had weird issues with the headband shape leading to a painful hotspot on the top of most people's heads, they had insane clamping pressure and, well, they were a bit heavy, so weight was a bit of a double edged sword. The MSR7b, while they feel inferior in the hand, they are definitely superior when they're worn. They completely disappear from my head after a few minutes, like the 58x do. The MSR7 are always THERE, you can't forget about them, be it because of the weight, the clamp or the pressure on top. The MSR7b just gently hug your head and, well, stay there. The headband, while it feels less plush than the old ones, is a definite improvement. It's wider and, well, has the correct shape (Note to Audio Technica : Most people's heads aren't stretched ovals). One last observation I have to make is that the b are ever so slightly smaller in size. Both the overall build, but also the cups themselves. And with my MSR7 having HM5 pads and my MSR7b using stock pads, the MSR7 feel humongous by comparison. Again, I like the originals' feel better, but the lighter weight and smaller size are a plus for portability, so you win some and you lose some.
Having said all that... The MSR7 are my babies. I've put too much work on them, so I may be heavily biased here, but I think I kind of like the sound of the modded MSR7 more. They're much more spacious, they feel absolutely luxurious with the huge leather pads, and the sound is significantly bigger in every direction. For me, that means one of two things. Either I keep the MSR7b as is and make them my portable cans (for which they're absolutely incredible), relegating the MSR7 to mostly home use, or I use the MSR7b a bit at stock, burn them in and then start messing with them. I guess I'll go with the second option because of multiple reasons. First off, I'm a tinkerer at heart, I want to mess with stuff. Secondly, what if I can do more or less the same things to the MSR7b and they become far superior to the old ones? I mean, they may look ridiculous with the HM5 pads, but trying can't hurt. Third, I'm dying to take a look inside, see what else they may have done, because I guess what I'm hearing isn't just a new diaphragm. I'm betting on either a different chamber design or different damping scheme, maybe with foam instead of felt. We'll see... I hope to find everything out! I'll make sure to post everything here.