if i recall correctly the hd518, hd558 and hd598 all use the same drivers. its just the design of the enclosure which alters the sound (hd518 is more bassy, the hd598 is more neutral with a better soundstage).
between 320kbps and lossless, any difference you hear is partly determined by how sensitive your hearing is / how much of an audiophile you are and also by how detailed the headphones you use are. the hd series is more smooth than detailed so you'd be a bit less likely to notice on them than say the dt990. you will notice alot more clarity on the hd series than say a $50 pair of headphones though.
if you're on a budget the hd518 is likely the best bang/buck. if you're looking for the most neutral sounding cans then perhaps the hd598 though if you want something which is very detail oriented ( though detailed can also be fatiguing to some) the hd880 will have more detail then the hd series.
Thanks for the input! I am using a pair of cheap headphones currently and I have never owned a pair of "expensive" headphones (above £100). I do wish for the added clarity in the tracks that I listen to. I know I havent had much experience when testing the diverse range on offer but I am starting to realize that the music often sounds muddy and some of the treble is lacking. However i dont want to sacrifice a "warm" sound when listening and a wide soundstage, as you said. appeals to me greatly. So would the hd 598s be the ones to go for?