OK. heres the thing, Rockbox is a software/firmware replacement, it can improve on the sound quality of a device but it can not do magic.
if a device has a bad opamp and dsp. rockbox cant do magic.
Rockbox on a clip+ or a Cowon D2 might sound extreamly well. but installed on a packard bell V500 or some other el cheapo player the sound quality will vary.
especialy if people try it on cellphones which are known
i cant comment on the hifiman, never heard it. but i have heard very negative comments from people whos ears i trust.
but i can comment on the kenwood HD series. it hisses. it sounds bloated and vailed. the interface is horrible and slow. nothing in the world justifies the price they charged for a new one (paid 200 euros for a 2nd hand 20GB one)
i was tricked by the fi-tards to believe it was pure magic when really its a recycled toshiba player which excells at nothing at all.
all i can say is stop parroting the sponsors trying to get you to buy 800 piece of junk just becuase it uses opamps which were used in high end equipment 10 years ago.
like new age medicine becuase its an ancient and from the east DOSNT MEAN ITS GOOD.