Grados would make a good example for what you stated. However, HD590s aren't exactly the best example for "flat" cans - putting together the frequency response from j-curve's thread with that of the HD580 from Headroom (the former was normalized to the HD580) it's clearly evident that the midrange looks quite different from that of the HD580. Instead of relatively flat mids with a notch around 5 kHz you have a gradual roll-off from the midbass to 5 kHz, then a narrower notch, followed by highs at "normal" level with better extension - not unlike the K240S (and similar to the HD250-II), but not as extreme. The result indeed sounds fairly close to better-quality loudspeakers (e.g. Jamo 707) - probably your impression of "flat" stems from there - and works well with classical, but it's not amazing that the mids can seem "sucked-out". Fortunately this characteristic is easy to EQ out.
BTW, the frequency responses of the current HD555/595 seem like (overall) better-controlled versions of the HD580's. Definitely cut from the same cloth. On another note, the smoothness in the HD650's FR (left channel) is quite amazing, pretty well controlled. Now if they made cans of this technical level with the older, more forward presentation, that would certainly be a winner...