Well, some of the best sounding amplifier and Dacs ever produced in terms of pure musical non fatiguing sound signatures are known as being exclusively tuned by human ear as opposed to curves and measurement produced by a computer.
And regardless of the measurements of the HM801 (which was tuned in comparison to a DAC costing over $15 K...), I can say that the Hifiman with balanced amplifier module sounds breathless.
It has an organic liquidity, non fatiguing sound signature, sound stage, imaging, dynamism... that no ipod (modded with an external amp or not) can come close to.
It can drive virtually any cans that will sound very bad on an ipod... and it will drive affordable portable headphones such as the ATH-ESW9 with an extraordinary touch of magic.
No, it is not flat sounding, nor cold or analytical. It sounds big, bold, rich, and renders voices in an extraordinary luscious manner (there is no nasal, smearing, or unpleasant peak).
It may not measure "well", it is tuned for human auditory pleasures by human ears with extensive knowledge of what sounds great based on much more expensive equipment that sounds even more fantastic.
What the HM801 achieved for such a relatively small device (in comparison to much more expensive/larger desktop/home system Dacs) is nothing short of stunning.
I know the HM901 is now available with supposedly better Dac chips and I have no doubt it also sounds superb but I know it cannot replace the HM801 as the sound will be a different presentation/signature and I love the HM801 too much to let it go.
On the other hand, I have let go of many ipod shuffles, nanos, and regular ipods.