I might be able to weigh in and offer my 2 cents.
I am an avid Cowon lover, having owned many Cowon DAPs in my lifetime. I still have a i9+ and C2 as well as being a AP100 owner.
In my personal opinion, Sound Quality is in the eye of the beholder... or should I say ear.
My journey with sound kinda goes like this (from earliest to current);
- Rockboxed Sansa Clips = Amazing value, but sound always sounded a tad 'dull' (even when EQ'd somewhat)
- Cowon DAPs = Far greater DSPs, the choice of finding a sound signature that you like is probable.
- Alien Shozy = Wow... complete transparent background, completely black, no hiss at all. You can't even tell if the device is on or off! This device now highlights how hissy Cowon and Sansa DAPs are, especially when using DSPs on either device. Sound from the Shozy is flat, balanced but engaging, kinda soft but airy, makes everything I threw at it sound great. But only supports FLAC and WAV which is just annoying.
- Hidizs AP100 = Black background like the Shozy, same kinda quality as the Shozy but a different sound signature. I prefer it, but it's really dependent on listener. The addition of an EQ imo trumps the Shozy as now if you have any records or albums that weren't recorded great you can compensate with some tweaking.
In summary I used to abuse the hell out of Cowon's DSPs (Jet Effect and BBE), but now I tend to hear it somewhat 'artificial', preferring a more neutral sound with just slight tweaks to EQ here or there to compensate for certain albums/tracks.
There's no denying it, if I play a certain track on a Shozy, although it sounds great, if I take that same track and play it on the AP100, I can get it to sound even better, even if it's just pushing or decreasing bass a couple of notches etc.
The EQ button on the AP100 has been something I have sought after for years, the only other DAP I know of to implement such a button would be the Cowon E2 (and its not the same kinda thing really).
FWIW I have abx tested (on foobar) FLAC vs 320 and hear no difference, I use MP3's on the AP100. I may try FLAC later on and see if I think I hear an audible difference between the two on the AP100 but I doubt it.
Hope this helps somewhat.