I have owned quite a few excellent DAPs including Sony and Cowon and I am quite satisfied with the SQ of my iPhone 6 Plus and I don't find the sound at all muffled which is reported here. Either the ones who are saying the 6 Plus is muffled have defective units or the music files are of poor quality.
BTW I have got Neutron sounding nice on my iPhone 6 Plus though my B&W C5 IEMs.
I managed to jailbreak my new iPhone 6 Plus. However, it looks like it's next to impossible to be unlock the dreaded EU volume cap as I cannot find the volume limit file in the preferences using iFile and I have tried using EU volume disabler and EUunlimited but the volume is still the same.
I'm very disappointed with Audyssey mp3 player on iPhone. I paid the full price to get the full version with all the supported headphone functions and now with the new update I have to pay an In App Purchase to buy something I already bought. This was such a great sounding app but now the developers are hell bent in robbing their customers. Not impressed.
The headphones I am using are Audio Technica CKX9iS which are very good IEMs. I also tested with B&W C5 and still the same. Of course the reason my 6 Plus may sound the same as my 5s may be due to the EU cap being on both my 6 Plus and 5S which I once read affect SQ.
I bought the iPhone 6 Plus and report that the SQ is the same as the iPhone 5S though one notch quieter with the ridiculous EU volume cap. There has been a jailbreak released for the the iPhone 6 and 6 Plus but I am unable to install it as it isn't available for Mac OSX yet.