518 the sound stage was much smaller and less clear but lows could be heard a bit better i think (not sure)
is there much of a big difference 518 having a low frequency response of 14 Hz while 558 have 15 Hz?
Honestly not sure if those numbers matter, at least not on my pair of 558s. They don't sound like they go anywhere near that low. I actually feel the bass response of my HD600s easily bests that of my HD558s. And my Fidelio X1s naturally have a boosted bass response in comparison to both.
The cheap cable i got from ebay definitely changed the sound of my hd518 for the better, it sounds a bit more clear and airy now, the bass is slight better too.
I got the same cable for my akg and shure headphones but i don't think i noticed any difference, if there's a difference it's very, very subtle.