While I sadly don't have much first hand experience, my personal belief is that balanced gear is like a more straightforward tube amp. Not every headphone can benefit from it, some may even suffer, but there are some that improve a lot with balanced. What people need to keep in mind is that a balanced design will usually work better when used in balanced mode. Be it due to lower noise, higher power, seperated grounds or whatever, an amp that is designed to be run balanced will behave accordingly. So I think that people comparing the SE and balanced outs on the same amp are kind of missing the point. It's much more complicated than "balanced is good" or "balanced is bad".
Well, take the 58x. Close them, make their bass a bit tighter, their soundstage a bit wider/imaging slightly worse, and just boost the highs until you're just above complete neutrality. Congrats, you now have MSR7b.
In all seriousness though, the MSR7b, apart from the obvious difference in treble are very similar to the 58x. So ask yourself if you could use a 58x with boosted treble and there's your answer. My take is that the MSR7b are among the best closed backs you can buy, period. Their driver, if it's any better than the MSR7, is amazingly capable.
A quick comparison would be this :
- Bass : MSR7b wins. It's better extended and tighter on the lows. Not a huge difference, 58x sound more mellow on their lows, MSR7b is more visceral.
- Mids : 58x wins. When you don't lose by a long shot to Sennheiser's mids but are just behind, I consider it a win. The MSR7b make me think the singer is 0.5m in front of me, singing for only me. With the 58x I feel like I can touch the same singer. It's got something special, but the MSR7b is damn good for vocals too.
- Highs : Depends. Are you ok with a bit of brightness? If so, MSR7b wins. Better extended highs, more of them, actually may be smoother. If you prefer a bit quelled treble, then the 58x's dark-ish signature is for you. Their treble, while very present, is never ever EVER offensive in any way. Can be boring with some tracks though.
- Details : I've heard 4-digit price headphones that merely trade blows with the MSR7, and the MSR7b is just as good as its predecessor. So yeah, poor 58x.
- Imaging : I've heard 4-digit price headphones that merely trade blows with the 58x's imaging. The MSR7b are damn good, but they're just not pinpoint accurate like the 58x.
- Soundstage : That's where things are rough. I'm not even gonna compare to the likes of the LCD-2C, because there's no point. The MSR7b has very good soundstage for a portable closed back, but that's about it. The 58x is really narrow for an open back. So it's kind of a wash. The MSR7b may be a tad wider, but the soundstage of the 58x is a tad more realistic, more believable, so I'm gonna give it to the 58x, but barely so.
Random pros and cons of each can :
PROS : (MSR7b) Portable, extremely light, extremely comfy, easy to carry around, doesn't leak much sound. (58x) Open, comfortable as ****, when music doesn't play it feels like you're wearing nothing.
Cons : (MSR7b) Can get sweaty, can get slightly harsh at high volumes, pads are rather small. (58x) High clamp pressure at first, pads are meh, stock cable is long as hell, sounds lifeless with some songs due to the recessed treble (in comparison to the MSR7b, not in general)
Having written all that, the comparison is mostly invalid because open vs closed and because they're both terrific headphones. Also, they both get their butts handed to them by my modded MSR7, so they have that in common too. The MSR7b may have the potential to become even better with modding, though, the 58x not so much.