Personally, that would be "as flat as possible, with no glare." You'll hear as close as to what was recorded if not intended as well, regardless of genre or era. If there's any part that can be (or left) modified, a little bit of wideband bass boost is acceptable, given the differences between speakers and headphones. Basically, look at the response graphs of the likes of HD600 and the K701.
More on that need for a little bit of bass boost. You might notice a lot of threads asking for "more bass," if not also questioning why those that have "little" bass (actually flat, but not totally - most of the better cans out there have a tipped up bass response over the midrange) are considered "hi-fi" if the music has a lot of bass. The problem there is how people understand how "hi-fi," recording, and their own biases/reference works. Let's take EDM as an example. The DJ's recording monitor does not necessarily sound the same as the speaker set-up in da club. In his own studio the DJ might actually use flatter monitors; or, he might be using monitors with a tipped-up response (like the better headphones described above), and so does not put in more volume on the bass instrument. In da club, which might not even have the same DJ as the host playing, the acoustic environment is nothing like a studio, you need the music LOUD so they'd be dancing (and thirsty, having fun, etc) in order to sell drinks; also, a heck of a LOT more bass to drive up the beat, getting them to dance and have fun, buying drinks and possibly ecstasy pills also (look up car bass set-ups on YouTube and you'll see why they're related). The problem is whether the person on the thread here pops pills or not is that the club set-up or his friend's Escalade is his reference for how a playback system should do bass. On a speaker system with a subwoofer even with a reasonably flat response can have enough bass to tickle your arm hairs as long as it's in the recording. That just doesn't happen with headphones, but the excessive amounts of bass that would usually give non-EDM/crunk fans a headache compensates for the most part.
It's the same thing with non-EDM listeners, just to a lesser degree. If your reference for the real music is a small venue where every kick on the bass drum can be felt on your chest, even from the bar across the hall from the stage, and then your reference for the playback quality is your dad's reference system with tower speakers with dual bass drivers on top of the midwoofer and tweeter that can also reproduce that "kicking" sensation, then you would look for a louder THUMP! on the bass drum on the headphones, sometimes even if it already affects the soundstage depth and puts it on the same point along the Z-axis as the vocals, if not in front. Similar thing with some car systems I've listened to - others have their subwoofer-midwoofer crossover setting too high that even a time alignment processor (delays the closer speakers to give the farther speakers time to reach your ears all at the same time) can't help how the bass drum THUMP! sounds "accurate" tonally as well as for that kick to the chest sensation, except it's coming from behind, which in reality doesn't happen when the rest of the drums are in front.
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Now, the real problem is that there is no perfect transducer that has an absolutely neutral 20hz to 20khz response, let alone one that's affordable. Sound signature really isn't a "specialty" as the term implies, but a compromise - if we can't make it totally flat, let's go for this kind of response curve at least - that the manufacturer prefers or can live with. Still, the same idea applies - as neautral as possible but with a little bit of wideband bass boost around 60hz to 150hz. You'll notice how generally the most popular headphones fall into that category actually as far as the graphs are concerned, but certain differences in certain regions of the response curve can make for what subjectively gets classified into different sound signatures.
