1. You need music with enough production quality and details...
2. You need good enough playback equipment (I don't know if PX100 really quality, I didn't like the PX200 all that much, didn't try the PX100). The iPod Mini should certain be enough given the right headphone to match up with it.
3. Your ears... doesn't have to be hyper sensitive or "golden ear" as they say, I never considered myself a "golden ear", but now I'm more aware of the differences.
4. Your brain.. even though you might be able to "hear" it, you might not understand how to interprete detail yet. I think this is true with most people who at first say, "There's no difference" then later detects a huge difference. Your hearing, like your eyesight, doesn't improve over time, they'll only age and get worse. Your brain learns how to interprete what you're hearing though.. You start to recognize, "Hey, that's an artificat" and "Hey, that's a badly represented tone for a guitar!".