598/600/650 "trio" ??
The 598 are unrelated to the 600/650...
Regarding the 70x cans - some claim there's *huge* differences, others that there aren't (AKG themselves are closer to the later category) - personally I find the K701 a nice headphone for all-around use, but they aren't "sweet" sounding. Not like the ESW9 or K1000. The A900X are less romantic/sweet/full-bodied than the ESW9, and lesser overall to the K1000 (the A900X and K1000 have bigger/better soundstaging than the ESW9s - accordingly). Their fit can also be temperamental. That having been said - they cost less than the ESW9 or K1000, and are not bad by any means (I'm not meaning to knock them). For the money, the A900X are a *very* good choice here. And on the upside, Magnolia will have the ESW9 and A900X side by side for comparison - so you can see which of the two you like. The K1000 are available from Audiocubes (and Audiocubes is a good dealer to work with). The K701 are available from myriad sources.
The K701 will do better for rock/metal than the K1000 or ESW9. The ESW9 will do best with female vocals specifically, the K1000 will be the best "in-between" of the three (they cost the most too).
The HD 580/600/650 are a good family to look into, the K1000 is easily described as a less-dark closed version of that family (despite being from another country, manufacturer, etc - it's kind of uncanny how they relate). I'd take the K701 over the Sennheisers for metal and rock, but the Sennheisers for pop or vocaloid.
Oh, I like Dubstep Girl's suggestion on Grados too!
Regarding the amp thing - worry about that AFTER you pick a pair of headphones, not before.