Don't know about the WS55i, but I own the K540. They have good, crisp treble, but if there is sibilance in a song, they'll pick it up and make it slightly worse than it already is, not a lot, just slightly. Mids are crisp and clear aswell, only lacking a bit. Bass is barely punchy enough for electronic music, does go quite deep and is fairly tight aswell. They definitely aren't lush sounding cans though; they're thin sounding, so they wouldn't be good for something like rap/hiphop that requires a lush/full sound signature, but they're especially good and engaging for the genres you're listening to. I wouldn't pay more than 60 euros for them though (around $80). Keep in mind, they need A LOT of burn in, atleast over 100 hours to sound at their best. They sound horrible in the beginning.
I compared them to the Shure 840, and I think the K540's have more clarity in the vocals, but the 840 has more detail and definitely a fuller sound. But tbqh, I think they're clearly better than 3 highly recommended cans here on head-fi (unamped atleast), which are usually found around the same price like the K540's (Shure SRH-440, Philips Citiscape Uptown and the Sennheiser HD 439).
Hope this helps you enough to make a decision.
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