MRT-Man
500+ Head-Fier
I personally prefer to use the kse1200/1500 when listening to metal because of their god level speed and detail retrieving.
I'm sure it comes down to personal preference, like any genre. But I agree, for metal, I value speed, details, separation/layering (since there can be a lot going on, depending on the type of metal), and mids/treble.
Dream Theater is my benchmark for testing, given the complexity of their music and the production quality of their albums. For speed, my torture test is the opening riff from "The Enemy Inside"; for details/layering, "The Gift of Music" and "The Count of Tuscany"; and for timbre/vocal quality, "Wither" and "Along for the Ride".
I also find the KSE to be a great option for metal. I also really like the Ragnar with the Ode to Laura cable, since it's somewhat comparable to the KSE in speed/detail/clarity etc. but with much better bass and overall tonality (the cable does an awesome job of taming the treble and adding more weight to the mid-bass & mids).