Hey cromag. The great thing about DIY and general purpose computation is that there is no "positioning" as far as as we the customer is concerned! You can use them for whatever you damn well please :-) In the commercial MP3 market, I suspect there is nothing in production that even has the very basic features I want, let alone any decent software polish or tweakability for itch-scratching.
The CHIP costs $9 sans battery, case, screen.
Re bulky: what would be your guess about which direction that is heading? Yeah, mine too :-) Somebody will be selling watch form factor PCs soon (though it remains to be seen what wired IO they will have, so far it seems it's not going away on the smaller boards). As of right now, take a look at the CHIP:
http://makezine.com/2015/11/28/chip-vs-pi-zero/
This kit could be rearranged with a 3d printed case to make something smaller. The screen is larger than needed, I think could be replaced with smaller device (haven't looked into that). The battery is not huge but probably is thicker than needed, again haven't looked into replacements.
https://getchip.com/pages/pocketchip
FiiO X1: looks like no MTP? That illustrates a problem: there really aren't "ends" of this market for me, only a small set of features I'm feeling the lack of, which it seems the market happens not to cater for. Also, with this kind of thing when it breaks (has been frequent for me), you have nothing and typically it's discontinued so you get to repeatedly spend time looking for a replacement which may be worse in various ways, which is an annoyance. Stark contrast with PC software - most of the software I use are the same programs as 20 years ago, but they have evolved to be far better in many ways than they were then.
The nice thing with general purpose hardware is that there is some continuity, so the software can actually get better with time (!), parts can be replaced when they break, and if a board goes out of production, you swap it out with a different manufacturer. Of course it may get hard to find ones that have audio (because bluetooth), but that should be fixable with a small addon module (e.g. USB) - and actually so far it looks like all that will be needed to add is the 3.5 mm connector itself, not the electronics. In principle physical board sizes could be a problem for swappability, but that may be dealt with either with a slightly over-sized case or maybe just the contined shrinking size of computer hardware.
Battery life right now with the PocketCHIP is tolerable I think though could be improved (it's designed for gaming and claims 5 hours, so taking both over-selling and the fact that screen can be dimmed for audio playback, 5 hours seems conservative target).
In fact having thought about it a little writing this, if there was PocketCHIP in stock (sold out, more due later Q1 apparently) I'd order one right now, start with some existing player like GMU (see below) and then maybe upgrade it stepwise as and when I get time and feel like it. Get MTP running. Find a smaller screen + cardboard and duct tape for my first case. Thinner battery. 3D print a case. Add a Python UI, or swap out MTP for something else, or patch it to fix irritations for podcasts / audiobooks / classical music, like per-track last-played point. I'll keep an eye out and who knows I may report back...
https://bbs.nextthing.co/t/media-player-gmu-port/6295/7