I've noticed the sound on my iPhone 6+ sometimes distorts with bass on EQ10 app which is my favourite eq app on iOS. However EQ10 doesn't distort on my iPhone 5S using the same EQ settings and sounds better. However when I use Audioforge Equalizer on my iPhone 6+ the sound quality is very good and it doesn't distort. Strange.
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.