Signal2Noise
Headphoneus Supremus
...
Most Android music apps I have tried on the X5 won't see the second SD card. ...
I have two DAPs with dual sdcards: this one and the Pioneer. I also use/try/buy a lot of music player apps. The only one I have in my app collection that does not see the 2nd card on either player is Rocket Player. Which is unfortunate since it is a really good player. Dev says they are working on secondary card support so that's promising.
I concur with the others Neutron is a solid app to use although the GUI needs an overhaul. I also don't mind FiiO Pure Music Player on the X5iii. In fact, if FPM were available as a standalone app I'd put it on some of my other devices.