OK, some testing done...
Sources:
Stock/standard nexus 5 and Sony nwz f886
Both UK models so will have to assume that they both have an EU volume cap - I have no way of veryfying other than the supplier of the Sony advising the EU cap would be present.
Both had the same flac album drag and dropped - so should be identical in terms of music offering.
Both used stock poweramp, no eq, no effects, just standard playback options. Also checked sound options within both sources settings were left alone and unchecked where applicable.
IEMs used: TDK ie800 and Sony xba h1.
Instant impressions were the Sony was certainly louder than the nexus when using the TDK iems so to he fair they were swapped back. In this instance (with the tdks) the Sony was still just louder than the nexus 5 but not by as much as the Sony iems. In each instance the Sony sounded better when presenting the music through the earphones and I think its fair to say offers a level or 3 above the nexus - which would be expected being a designated musical player
I switched to the stock app on the Sony and IMO this is better than poweramp. To me and in my (limited) experience it delivers a better all round and more enjoyable sound
Hope that helps