Bumping this thread to add a comment.
I got some Senn 600's and I must say, it's an interesting comparison. Firstly I realize I am comparing a $400 to a $200 headphone, but I think the HD600's are worth more than their asking price nowadays. I've always liked them, just never owned a pair before. The HD600's treble is really rolled off compared to most headphones, and Grados have slightly amped highs compared to what I consider neutral and this manifests itself in different ways. The thing that surprises me the most is just how much the old stereotypes are true. The Senns sound worlds better with symphonic band and classical music. You can crank it and crank it and they're never fatiguing. On the other hand the rolled off highs make rock music sound really dull. That lost treble, for me, literally makes drums sound like they have less impact, like the drummer is playing on a cheap drumset with loose heads or isn't completely with the rest of the band. That's got to be why I've always thought of these as sounding "slow". It also makes the sound seem a lot less detailed than on the HF2's. The HF1's with flat pads sound a lot like the HD600's, actually, but they still smoke the 600's on rock while the 600's smoke the HF1's on loud orchestral, band, and organ music, having what I perceive to me a more even high-frequency response (then again if you lower the whole upper end of the spectrum maybe that effect is inevitable).
The thing about the HF2's that detracts from an otherwise nearly perfect headphone in my opinion is the funky midrange. These have some weird peaks in the upper midrange that make it sound like you're listening to JUST THE VOCALS through a poorly designed horn speaker, and only at certain times. It's so weird. I've heard wavoman's HF2's, and he doesn't use his as much, and the problem is horrendous on his pair, in fact, the midrange sounds really really bumped up on his pair of HF2's. Clearly the problem goes away as you use them, or there's manufactuing variation. Who knows, maybe I was having a bad day when I listened to his, but to me it sounded like the midrange was blaring. Regardless, the midrange on the Senn HD600's is a real selling point for them. In terms of sheer evenness and accuracy my two Grados can't touch them, and really, I doubt many headphones in this price range can compare. It simply sounds natural.
Oh, and the HF2 really is 90% an improvement over the HF1. You really can't compare the detail between the two, it's like comparing an SR60 and an HD800. The HF2 really does have much better resolution in the treble than the average Grado. The 10% that isn't an upgrade is the fact that the bass on the HF1's is pretty gratifying, and the weird midrange on the HF2 that isn't present on the HF1 at all.
Yeah, the HF-2 do have an upper mids peak compared to other phones, but you do get used to it the longer you listen to them.