I was very recently on a shopping trip with much the same lineup (plus a few others) so I'll add my two cents. I found the 598 an excellent headphone, superbly smooth and even-handed in its presentation. Comfort is great too. I will say that it doesn't quite get the level of sound resolution as the other phones here, but hey, music is about the emotional experience, not the specifics of the gear. A really excellent headphone IMO, and price is a plus. The 650 is a little laid back unamped, but even unmapped it still resolved impressively, had a natural feel. Definitely a can needing amped though, so keep that in mind. I would not buy ortho dynamics until you've tried at least one pair at any price point. Orthos have a bit of a "fady" distant sound despite their superb resolution and precision. Some people really can't get into The ortho sound. I would recommend looking at some ZMF headphones if orthos are something you like though. Haven't tried the Allesandros but they are supposedly more "subdued" than the regular Grados. I sprung for a pair of Sr325i until I can afford an LCD-XC but if the senn 650 had been cheaper I would have gotten it. My biggest piece of advice is really try before you buy if at all possible. Frequency response and dialogue ultimately means nothing when you out your ear to a can. I think the Grados are less comfy than the others, but I built a little basic headband pad for them (Velcro, some sewing, pretty simple) helped a lot, but I find them alright stick.
I will say that I tried the tape mod and noticed no difference, but maybe I did it wrong.
Trios, rock, small and intimate things, fun recordings, the grado works wonders. Mind you that people who say they sound "bad" on something are saying that in relation to other cans. All the products at this price point won't be "bad" at anything, just better at certain things. For the money you're paying the headphone should be capable of anything, and then it will have specializations.