A little update:
So, following an unexpected bonus, I bought the Sony MDR MA-900s new, and a very lightly used second-hand pair of HD600s.
The Sonys arrived about 10 days ago, and I've been using them constantly since. As promised, the soundscape is startling wide, with sounds popping up all over the place. The width of the soundstage can actually be a bit distracting at times, seemingly slightly removing presence from the centre stage. There's quite a bit of detail in the mid-highs, with some pretty smooth highs. They aren't bass monsters, but that's okay. I've really enjoyed listening to all sorts of music with them. They've very easy to drive. I rarely go more than one or 2 "notches" above zero on my iMac. The comfort is a bit weird though. They're very light. The earcups are VERY comfy indeed, sitting spaciously around my ears. The headband is a lot narrower than the wide, thin band on my Grados. It's very nicely padded, but since the pressure all seems to be located in a relatively small area, I definitely feel the pressure on the top of my head much more noticeably. This is undoubtedly and unfortunately annoying - not in a rage-inducing, defenstrative kind of way, more of a little itch that you have to scratch occasionally.
I just received the Sennheiser HD600s today, and I'm only driving them directly from my iMac thus far. However, even with that relatively weak source... oh my god. These are incredible. I can definitely see why they're commonly referred to as gateway phones. I'm currently listening to one of my favourite albums, Arvo Part's Tabula Rasa, and the clarity is just something that I haven't heard before. It feels like every delicate little nuance of the sound is given space to breathe. Part of that is undoubtedly the gorgeous, spacious arrangement, but it even works on much "hotter" pieces like Alt-J's An Awesome Wave, Aphex Twin's Richard D James LP, and Animal Collective's Merriweather Post Pavillion, Black Moth Super Rainbow's Dandelion Gum, and hell, even Jay Z & Kanye's West's Watch The Throne (it doesn't get much hotter than Church In The Wild or Ni**as In Paris). Back on tracks with more dynamic range, William Basinski's Watermusic 1 burbles along beautifully.
Comfort-wise, they're a little clampy, but despite being much heavier than the Sonys, the greater adhesion to the side of my head renders them much lighter on the top of it. Unfortunately, the earcups are a smidgen too small for my enormous earflaps, though it's not uncomfortable at all.
In short, I'm sold. Totally. Now I'm researching headphone amps and stressing out about lossless audio codecs. Dear god, what have you people done to me?