Lucky for you on anyone reading this, I've already found a good solution earlier this year. After at least a full day experimenting and trying other music players on the DP X1A, I settled on the
PlayerPro from Blaston. It's a free player and they may have had an add on for $5 (though I don't recall). You can download it from
http://aplayerpro.com/ or the G-Play store. Be sure to also download the DSP Widget pack and follow the instructions, which enables a 10-band EQ as well as both a stereo virtualizer and bass boost knob, both of which are actually quite useful with IEMs and actually quite good quality, and allowing you to choose between two different processing algorithms. Speaking of options, PlayerPro's options are second to none, and my testing has shown me their (upsampling) setting of 48khz SoX Resampler, no dithering and 32-bit output to have the best fidelity. It also has options to auto-stop on headset disconnect and auto-resume on plug-in, though resuming also necessitates pressing the wake-up button. Its' navigation is my favorite I've ever tried bar-none! You can simply navigate to any directory where your music is stored, then it will automatically play-through the current and all adjacent and sub-folders, in the correct order, and
ALWAYS resumes precisely where you left off to the same second within the same track, regardless of whether you've closed the app, rebooted, etc.

It also boasts options to sort via artists, albums, genres, folders, and allows you to add favorite songs to custom playlists.
My main reason for going with this app was because of sound quality. Extensive testing I had done using the same loop of a well mastered lossless song on both good headphones and earphones with this DAP had shown me that the SQ of PlayerPro is at least 95 if not 99% as good as the included player. I also compared these to the Poweramp player which actually is compatible with the DP X1A, however multiple A/B tests have shown me that
the sound quality Poweramp puts out is GREATLY inferior to both the PlayerPro and the internal players (don't recall the precise differences but it was something on the order of less detailed, less dynamic, etc.). I also tried the Winamp app but don't recommend it due to extremely minimal functions.
(not my presets, LOL)
Next I'll list the negatives - having already used this app for several months I've noticed a few hiccups: The physical seek / navigate within track buttons are not compatible with this app, so you're left with only navigating within the touch screen (though for me I've come to realise this method is more intuitive anyway). For file types it plays everything except DSD & MQA. I've had no problem with that personally, being able to simply alternate between this and the built-in players when I want to hear DSD tracks; preserving both playlists. For playlists it either doesn't have an export feature, or I haven't figured out how to use that yet, but at least I can favorite my tracks from a playlist within the app.
Finally I want to bring to light a "hiss" issue that I've been noticing. I'm actually not sure if this is caused by the PlayerPro app, file types or this DAP itself, but when using either the "normal" or "high" gain settings,
I often but not always hear a sort of hissing sound during semi-quiet parts of the songs.
It seems to be a software issue either in this app or its' settings, or an issue with the DAC section itself of the DP X1A - that is, rather than the amp section because when a song is paused the hiss disappears. It is also not part of the song itself which shows a black background when played on other systems. Further testing is required to get to the root of this issue, but the issue seems to be very recording-dependent and is noticeable less often.
Does anyone else notice this same issue, whether using the built in player or an external app? If so, which file types do you notice it with?
Nevertheless this has become my go-to player and I don't want to replace it.