Long story short, I owned the HD 555's for about a year and a half and loved them until my bunny destroyed the cord and I decided to gift them to someone and try a bit of an upgrade. I have a budget of $250, and was looking at the HD 558/598, but was mainly wondering whether the 598s are "worth" the $70 price increase. Also briefly considered something like the HE-300, but I don't know. I don't have an amp and will be listening through my desktop's motherboard. I listen to almost exclusively Vocaloid music (for example, http://www.youtube.com/watch?v=ba1vgZRT2K4 and http://www.youtube.com/watch?v=wf4D6e11omU), and a select few English artists like the Script and One Republic. Are there better headphones for these kinds of music in this price range? Would also be a plus if they're decent for gaming. Also willing to consider an entry level headphone/amp combo if it's under $250.
If you liked the 555, you will love the 558/598. Get the 598, especially if you plan do any serious gaming. Both are very efficient, but there is no way to guarantee that they will be properly driven by on board audio of computer X.