Really bad sound using non-Apple ear buds

A few months back something happened to the Apple earbuds that came with my iPhone 4s (I think my roommate swapped his bad ones for my good ones). I have since purchased a couple different brands - Ear Candy and Sony, to name two - but EVERY ear bud set I use sounds "tinny" like I'm listening to my music through tin cans. Is the iPhone designed for optimal sound quality only when I'm using Apple earbuds? I have set the EQ on the phone for "bass booster" and alternately for "treble reducer" and nothing seems to work. When I plug the phone into my car stereo the sound is great. But with ear buds, not so much.

Suggestions?