I just finished breaking in the 518's I ordered for my wife. They sound IDENTICAL to my 598's.
I've had a half dozen people listen to both including my kids who have excellent hearing range compared to me. Only one person thought the highs were ever so slightly clearer with the 598's. Mind you those are a much nicer set to touch. The materials are better, but not enough to warrant the price difference.
I'm not upset i bought the 598's by any measure but others should be aware of the distinct lack of sonic differences in these headphones.
Both work great without an amp, but both do sound fuller with one. It's a noticeable but small difference.
I very highly suspect the 558 sound identical as well since they use the same drivers from everything I've heard. Please correct me if I'm wrong.
And yes, sound stage is no wider on the 598's.
I've spent 10 years in a past life doing SQ car audio, talking nothing under $5k installs here. I have no illusions something costing more will make it better. I don't judge based on cost or "expected results."
And I can assure you, an expensive USB cable is 100% a waste of money.
The HD598 has significantly wider soundstage, and is more crisp and airy while the HD518 has a fuller bass and more muddy highs but overall the smoothest sound. I remember that when i switched the HD518 for the 558 i was quite stunned at the amount of difference and refinement. The HD558 has a less airy sound, more impactful bass smaller soundstage (though still wide) than the HD598 which has a wider sound, lighter tap on the bass and more focus on the upper mids. Overall i'd say the HD518 has a significant drop in performance due to it's semi open and poorly ventilated construction and much stiffer earpads.
(but with the modded HD558 that I have with the inner lining, foam, grill cloth and also the silver ring taken out, it is 100% the performance to the HD598. I am confident enough to say that after having done A/B tests.)
also: Well I know of someone who says that his USB cable made a difference
Edited by streetdragon - 10/23/13 at 11:10pm