I find quality in-ear headphones extremely revealing of poor quality output. Shure e5c, to pick an older set that I own, are especially unforgiving of noisy output stages and cross-coupled interference. The additional acoustic isolation really helps your brain focus on the audio by cutting out the masking effects of environmental noise. This isn't always a good thing.

For example, the Cowon S9 audio player was supposedly designed for high output quality, with a pile of DSP presets for EQ etc. With the e5c's you can really hear the poor quality processing of the EQ, especially on reverb tails. Have to turn the EQ off completely to eliminate them.