This is part 3 of my testing of music players on Android. My first post is here http://www.head-fi.org/t/638387/best-android-music-player-app/420#post_12607442.
I spent some more time with the players today and decided to update my table.
USB Audio Player PRO: I was able to test it by following the suggested link. It has the ability to output high resolution audio to a USB DAC, just like Neutron and Onkyo HF do. Besides that, as much as I looked I could not find any single feature that separates it from the rest of the players, which makes me question its price. I also did not find the interface superior in any way. If anything, I would say I prefer using the majority of the other players in the list.
Rocket Player Premium: The free version does not have gapless playback, but the premium version does and it works. There is nothing that makes Rocket stand out from the rest of the premium players, and for the same price I would definitely prefer GoneMad, PowerAmp or jetAudio Plus.
Since Joe posted above that on a rooted phone it is possible to export and edit presets of the equalizer in Onkyo HF, I gave it a green square on the EQ as this makes it one of the most capable ones. It does require a root however, so the red mark for equalizer export still remains, as it is difficult and for some phones impossible to do. Also I went back to some of the other players to look more closely at their EQ bands. Most of the players that have 10 bands have their frequencies adjustable at 31, 62, 125, 250, 500, 1k, 2k, 4k, 8k, 16k, except the following:
Stello (12 bands): 30, 50, 90, 160, 300, 500, 1k, 1.6k, 3k, 5k, 9k, 16k
Deadbeef (10 bands): 60, 170, 310, 600, 1k, 3k, 6k, 12k, 14k, 16k
jetAudio (10 bands): 60, 100, 240, 500, 1k, 1.8k, 3.4k, 5.7k, 9k, 13k
jetAudio Plus (20 bands): 30, 60, 100, 160, 240, 350, 500, 700, 1k, 1.4k, 1.8k, 2.4k, 3.4k, 4.4k, 5.7k, 7.2k, 9k, 11k, 13k, 16k
Depending on what adjustments are more important, this little detail could be important to some people. I find an extra band above 1k very useful for my particular earphones when using jetAudio. I would definitely think about it if considering buying a player that has the regular 10 bands.
After about a week of using jetAudio one thing that I have come to appreciate is its ability to update its library by using the native media scanner that is built into Android. It is the most elegant way to do it, and it should work great unless wanting to play files that are not natively supported by Android. It should have most people covered, as FLAC playback has been in place for a long time now. USB Audio Player PRO also has the same capability. Android's media scanner can be restricted by placing an empty file named .nomedia in each folder that is to be excluded from the media library.
Update:
I tested today AIMP player and it seems that it is also using its own sound engine and not the native android's and I added it to the table above. It falls into the same category as DeadBeef and ArmAmp as it doesn't have a library, but it is free without ads and in my opinion with a preferable interface to the other two.
Below I'm including another table of all the other players I tested. They all use android's native sound engine, not FFmpeg decoder, so they have very limited or no equalizers and no gapless playback (unless the phone supports it natively). Xplay is an exception, but it hasn't been updated in a long time and gapless playback as well as some of the functions in the interface did not work correctly on my phone. BSPlayer, MX Player, and VLC are all video players and do use their own sound engines for decoding sound, but they don't have built in capability for gapless playback as they seem to be designed primarily for video playback at the moment.
I have colored Google Play Music, Shuttle and Vanilla Music as three players that might be worth taking a look at. They are free without ads with a somewhat preferable interface to the rest and all three will play gaplessly on most phones with android 4.1+. Google Play Music has the benefit of being preinstalled on most phones, Shuttle is lighter than Google, being just a local music player with a basic 5 band equalizer, and Vanilla is among lightest players available that is also open source with a great interface.