What you want to do is load the same file/song onto the internal and external memory, (preferably something with strong vocal presence) because that's where it's easier detectable, then switch between them listening just to the intro, say the first 10 - 15 seconds from the headphone out at a decent volume. Listen for the vocals, I find the internal memory sounds more dynamic and there's slightly more detail, you will hear deeper into the singers vocal notes, I did this in a dead silent room laying down, the amount of times I went back and forth almost drove me crazy.
When you amp the line out though the problem is much less obvious to me, in the end I just forgot about it as the majority of storage space wins, even stranger after a month or two when I'd switch back to play the crazy game I enjoy the external memory as it feels more airy and less congested. Maybe I just convinced myself that was the case? I don't know, we can't narrow out the possible cause my unit being a lemon either.
It's a strange one, that consumed many hours of my listening time. Edited by H20Fidelity - 6/6/13 at 5:14pm