they were on sale for 100$ and i wanted something light so i just grabbed them. Was thinking of returning them and get the HD558.
Pretty sure the 558s are identical in sound to the 555s, and I'm enjoying my Monoprice earbuds a hell of a lot more than my 555s. Even with stock tips, they're that damn good for music. Gaming is good too, but I think the 555s has better spatial (positional? forget the correct term) sound so depending on the game I might switch them out.
Here's a comparison with the HD598 included.