Comparing to pretty much anything I've had in the last few years (xonard dgx, stx, g2, g4, mojo, pureview 808)
Don't get me wrong though, I love my s7. It's snappy, great camera, decent battery, versatile, vr etc. I've had it since the release.
I purchased MSR7 (fantastic headphones btw) a few weeks ago as my out and about headphones and thought that my s7 may not do them justice. So my portable DAC/amp quest started. I did some A-B tests on some "quiet" evenings and compared xonar essence stx (desktop), dragonfly black and mojo with s7 (on poweramp). I was using my few weeks old msr7 and fidelio x2. Played a number of songs mostly in flac, various genres (rock, metal, classical, house).
S7's sound quality is in a different league (B or D league to be clear). I personally don't care what dac/amp it has it just sounds poor. Keep in mind that prior to that I was using s7 as my portable player so my ears were biased/conditioned towards s7. Even so it didn't help s7. I'm not an audiophile so can't describe s7 sq in technical terms but the closest adjectives I can think of are, flat, tinny, lack of details, lack of power (90-100% volume required), tinny, no instrument seperation, poor soundstage. It was probably unfair to compare it to mojo as it's £400. But even in comparison to 808 (which is good only for photos and music) s7 sounds like a cheap chinese knock-off. That's really unacceptable for a high end phone costing £560 (when released). Not sure if it's samsung's idea or android shortcomings. Good thing is that it plays high quality songs flac etc. Anyways, I picked £90 dragonfly black which brings buckets of energy, details and soundstage. I feed it with my s7 using usb audio player pro (£5). Seriously, try it. It's night and day difference. I also though that S7 sound was good but you don't really know until you try something else. If you after good sound quality £90 is quite reasonable for the huge upgrade you are getting and it doesn't add much bulk/weight (unlike mojo). Just try anything (dac/amp) or don't if you happy with s7 sq