The treble is less harsh on the Mad Dog, but it's not lacking. There's no spikes causing sibilance, so it's actually pretty nice. The HD598 is harsher, but brighter as well. I still like the HD598's treble, but the Mad Dog is much easier on the ears overall - the HD598 has a tendency to be sibilant in certain songs whilst the Mad Dog has only been sibilant when the recording itself is.
Also, It may just be my ears lying to me, but the new revision (of Mad Dog) seems to have better imaging than the previous revision and is just a little brighter.
Edit: It's nice to see the measurements page supplies more reference graphs now. A little thing: You don't specify what LCD in the description preceding the LCD-2's graph (it is mentioned in the file name and can be deduced from the price, though)
That's good. Do you find the bass to be an improvement as well? I'm not unhappy with the 598 bass, but I'd imagine these to definitely have better extension (especially from looking at the graphs).
Are these headphones being constantly revised? Should I wait a while before ordering them?