when you think MP3 sounds OK...
You're all so very smart. Digital equipment throws an awful lot of crap into the environment, and more so through the cabling, chassis', power supplies, etc. It interferes with the really quite delicate analogue signals in the DAC and the Amp, ie the stuff you're actually listening to. Different digital equipment, especially storage with all its electromagnetic interference, sends out different degrees and profiles of crap. A lot of work has been done to measure this. You can hear it if you bother listening. But you probably don't. You think your iPhone sounds OK.