I can answer this for you as well, since I'm also a metal fan and I have auditioned and owned various Grado headphones in the past.
Basically, the typical Grado headphone is more colored than the DT250 and the Grado coloration happens to suit most metal fairly well. Metal tends not to have a lot of sub-bass (although there are exceptions, fe. djent) so Grado's rolled-off bass is not a huge issue when it comes to this musical style. Grado also has a mid-bass hump that works well for metal as it gives kick-drums more slam and impact, which is another trait that metal fans appreciate. Grado is also known for clear and in-your-face mids and treble, and there is a consensus that this makes guitars and vocals sound particularly energetic and natural. The downside to this is that the hot upper-mids and treble sometimes results in high levels of sibilance, particularly with badly recorded albums (and a lot of metal is very badly recorded).
The above description applies to all Grados in the Prestige series, as they all sound extremely similar to one another, with the higher-end models refining the sound slightly over the cheaper models, but without changing the general sound signature. Grados in higher series (reference, signature etc) vary in sound a little bit more, but the majority still retain the above sound signature.
By comparison, the DT250 will sound boring, as there is no mid-bass hump and the mids are neutral as well. The DT250 will sound thin compared to a Grado as the bass is very neutral and doesn't warmify the rest of the frequencies. The trebles of the DT250 are just slightly emphasized above other frequencies, but not nearly as much as in Grado. The soundstage of the DT250 and the typical Grado is actually rather similar since Grado has a rather narrow soundstage for an open-back headphone. The DT250 is not particularly prone to sibilance (at least to my ears), even though it is a fairly bright headphone. Grados are way more sibilant and fatiguing than the DT250. The DT250's bass is much better extended than the typical Grado, the DT250 is very good at sub-bass frequencies. With all this said, the DT250 is in no way a bad headphone for metal - it has very clear, detailed and natural mids that are not recessed in any way. Vocals are very clearly distinguishable over music with the DT250, which is useful for metal since vocals often get drowned out by the bombastic music in this genre.
To summarize - Grados are colored in a way that suits most metal and makes it sound more energetic. The DT250 is a very flat (as in the frequency rate curve is flat) headphone and does not add any additional energy or character to the metal that is being played through it. Nevertheless, the DT250 is far more neutral than any typical Grado and sounds good with any genre you throw at it, whereas Grados don't sound great with some genres (particularly bass-heavy genres like electronica and hip-hop, as well as genres that require a neutral, balanced sound signature, like classical music).
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Thanks guys you've been very helpful!
@donunus I see you have metal as one of your preferred genres and that you have auditioned various Grado headphones which have consistently been praised as the "ultimate" metal headphone. How do the dt250's fare against the grado line for metal (I know open vs. closed is kinda like apples and oranges) and if you prefer them (which you seem to) why is that? I've auditioned some low end grados and found them quite enjoyable.