No clue. Haven't messed with iTunes at all. I know what works with my factory headunit and it's a pretty universal format for everything else.
As for flash drives I swear by the Sandisk Cruzer Glide drives. The 32gb is like $8 each and they don't get hot with doing a marathon file transfer. Probably because the actual drive is encased in thick plastic, lol. Either way, never as much as a hiccup out of any of them. PNY makes Baby Jesus cry in my experience. The larger profile of the Cruzer gives my short stubby fingers something to hold on to while threading the needle into my usb port.
pretty basic setup here its usb or bluetooth into the headunit. kdc x399 does a full 4v through all 3 outputs. turns up to max volume with no distortion. can even turn up the subwoofer out gain one notch without distortion. all tested with smd dd1. i have the extra notch saved for tracks that i know have -1db bass in them. for everything else my system will tolerate the full brickwalled 0db bass electronic tracks that are common and stay at <1% thd and rms ratings of the subs and speakers. no eq is ever used. did all eq though speaker selection, placement, and high/low pass filters on the amps set by ear.