So far, no problems with my new M-100's. No structural issues at this time and sounds pretty good. I've been messing around with the EQ on PowerAMP on my GS4, we'll see where that goes. If anybody has expertise or advice on setting a general EQ on the M -100's, please let me know.
I'm using PowerAmp on a Cowon Z2 and I find the techno pre-set sounds best to my ears. That said, it is sitting on top of Cowon's Jet Effect engine, so you probably won't get the same experience on an S4.
I'm also using it with a Fiio E12, though it doesn't change the sound signature, I just love the bass boost and the higher gain as my Cowon doesn't seem to have very high volume levels.