Originally Posted by dontpanicbobby
As I said I'm not going to argue. Just try this simple test: Grab any iPhone or iPod user you know. Give him a copy of your favorite song in MP3 format. Listen to the music on his or her device and compare it to yours. I would go into the whole Bluetooth Stack comparison thing but that's not what this thread is about. tjm276 wants some advice on a good music player for his S4.
As I read it he is talking about not being happy with the quality of bluetooth audio, the question about third party players is in relation to that and whether they might fix it. If you read the thread title this seems clear. For bluetooth give me an S4 and Apt-X headphones over an iPhone any day, if you're unfamiliar with Apt-X Google can help.
For audio via the headphone jack I agree, quality is better from iPhone, I don't need to do your test, I've owned iPhones and I wouldn't argue that either (notable exception being the Note 2 which is on a very nearly even footing, only bested by the iPhone on stock software because its volume output isn't as loud). I'm not speaking to audio out of the headphone jack though, only to bluetooth audio which is what the OP is asking about.
Third party players won't fix bluetooth audio, or headphone audio for that matter, but ones like PowerAMP, Neutron and GoneMad Music Player with good EQ options may help the OP get a sound they can live with.
Incidentally OP: if you were also talking about headphone out audio quality may I ask what headphones/IEMs you are using? The i9505 has a serious problem with lower impedance IEMs (less than 16ohm impedance) where bass frequencies cause crackling once you get to higher volumes. Fix there is lower volume or neuter bass in EQ, neither of which will be very acceptable to many (or getting higher impedance IEMs or an impedance adapter). Unknown at present if this is a hardware or software fault, although a clever chap who has measured the problem is leaning on hardware being the problem sadly.