First impressions, coming from my main K701s:
holy darkness, Batman!
And I never thought I would ever say that about Grados. I can hear the traditional Grado aggressiveness in the mids (oh how I've missed you, guitars), and the bass (probably more accurately, sub-bass) is there in much greater quantities than on the K701, but the treble, while probably somewhat balanced, is just completely overwhelmed by the mids and bass. I read some of the reviews and I don't think I could say that the HD650s are darker than the HF2. Honestly, I just can't see it. Seeing as how this is my first experience with bowls (my only other Grado experience coming from MS1000is and SR60s) I'm still getting used to the fit and the strange soundstage (the drivers are sitting on your ears, but there's still a little distance between where the soundstage starts and your ear... not to say that the soundstage is that large, but it's not quite as upfront as Grados with comfies).
Detail is there, but nowhere near the level of detail present on the K701s/DT880s. But then again, Grados were never really meant for analytical listening - they were made to RTFO, and that's exactly what I'm doing. Audiophilia can bite me, I don't want to analyze my music or my headphones, I just want to listen... the HF2s will stay on my head a little while longer
Point of interest - has anyone tried putting jumbo pads on the HF-2s? Given the darker sound signature I have a feeling the brightness jumbo pads usually impart upon Grados will work well.