The Cowon S9 is the least flat DAP I've heard, and it felt a bit like a toy, so plasticky and light.
If I recall, according to RMAA is measures like a flat line, when all eq is turned off (if that's possible), however the signal still sounded a bit fake, I think there was some processing somewhere, or a hardware filter of some kind.
The early firmware version of the S:Flo 2 / Teclast T51 had high-end roll off, perhaps to give some kind of analog sound? Anyway they removed it, and everyone seemed to like the flat sound better. I've always liked the Teclast T51 for it's reference, pure signal type sound, with OPA2604 and dual Wolfson's, it was refreshing compared to the Cowon S9.
These days the T51 sounds a bit lacking compared to my desktop audio, but it's still nice, no complaints.
The Hifiman HM-601 using TDA1543 and JRC5532 (I think) does not sound flat. Firstly, if you're used to modern digital audio (who isn't) and listen to the TDA1543, it sounds like a joke, it's so lacking all over the place, it's like you just put on some horse eye blinders, the first time I heard it I laughed.
After a while you can appreciate the 'sound quality' it exudes, which people refer to with words like analog, sweet or natural. I liked it with my Sony EX600, until the battery died after 2 1/2 hours on a 5 hour trip. Still, it was my first experience of NOS and it was pretty nice, I'd like to try a really high-end NOS desktop unit soon, if they can intertwine the natural sound of the HM-601, and the higher resolution, refinement, crystal imaging, more extensive soundspace etc. of modern DAC chips like my AKM or Sabre.
So on sound quality levels none of these DAC's are 'flat' in hardware terms, the T51 comes the closest I think, on software levels almost all DAP's are flat__________________________, which you can see in RMAA, however some DAP's will have secret sauce somewhere, I suspect Cowon of this, and I'm curious why everyone says the new Sony Z1070 sounds so good, I mean... can it really sound that good? I wonder if it has secret source, or if it's pure high-end performance from Sony, need to find out.
The T51 (and possiblly Hifimans) have/had software high-end roll-off, which is a bit bizarre to me, however Teclast fixed it with the firmware update.
There is still a secret sauce in the T51 though, it's called dynamic compression, you compress the music into a thin volume level and lift it up, this gave it an artificially high volume level, and less dynamic range, on the "Normal" setting. The thing is, a lot of people think compressed dynamic range / loudness sounds good. I noticed on my stereo receiver I quite like the compressed dynamic range mode, I turned it off now but it sounded good at the time.
Try the "Dynamics Compressor" in Foobar, set input gain to 20% !!
Long story short, almost all DAP's and DAC's measure flat, yet none of them sound alike. A valley or peak in frequency response will create a different presentation, but it's only an adjustment in volume of the fundamental sound. That's all FR actually is, volume dispersion.
Output impedance will have various effects on frequency response, depending on the IEM or headphone used. For example, my Sony XBA-3 sounds very shrill with 32 ohm output impedance, the XBA-4 even more so, this is due to the delicate design of the BA's inside of them and their low impedance specs (12 and 8).
The TDA1543 chips have high output impedance, which the Lite DAC-AH accounted for with AD847, I don't think the Hifiman's did completely, so they don't interact nicely with all sensitive IEM's, and by nature these DAC chips have noise or hiss (which you have to overlook for the NOS sound experience).
With all that said, a JH Audio JH13 user tried my Hifiman HM-601 and liked what he heard, I thought he was joking, but he really liked it. In audio, sometimes you have to mix light and dark, high performance with low performance, wet and dry, the intertwining of these concepts can sometimes result in a 'complete' sound. You need to mix treble with chalk, and poison with gravy, err..... french fries with happy times!