It doesnt really matter I believe, he is only going to get 48khz out of his iphone.Unless he uses the Apple camera connection kit and inputs to the Hifi-m8 usb-B.As of now he is just using standard cables.Imho.*please do correct me anyone if i am incorrect here.
Lazy link for you rudi.
I found the original venturecraft cable to be quite revealing (i havent tried their newest uber cable yet), i found the v-moda to be muddy, just how you describe your usb experience above.
I pair my Dita with the Hifi-M8 (idevice version) using ipod touch 5G -Flacplayer app and Venturecraft lightning cable.
The great thing about the Hifi-M8 is it has three impedance switch settings 10/2/1.
I can experiment with different settings and iems/headphones easily, it is not as extreme as I once believed.
DITA - still lovin em!