Frequency Response - on par with the HD600 - the only caveat being the presentation of the bass. It seems to DOES go lower, but not as much in Quantity as the HD600 - Listening to Eric Clapton Unplugged, the Presentation is warmer on the Sennheiser, but Warmth and Extension are two Different Concepts - Some music requires Warmth, Some Music Asks for Extension. As for me, I like both flavours.
Treble is Close to Grado levels, but far smoother and enjoyable for longer periods - Guitars are well-voiced, as are other instruments, but I now realize how special the Grados are for Rock-nothing else gets that texture out. Still, Pink Floyd and Diana Krall sound wonderful. Floyd's Dark Side of the Moon (live) shows the Cans' real Chops - on the Track "Great Gig in the Sky" the spoken bits in the beginning are much more coherent, the wailing is also more...human.
The Cans are very Revealing, and will not magically add tone to recordings with lean bass and midrange. Poor records will sound bad, and nothing short of an Equalizer (or a Very Good Preamp) will do.
Imaging - Beats the HD600 - the soundstage does indeed extend beyond the headphone in a smooth, sweeping arc across your ears. There is a natural depth to the Soundstage, and it is easier to place individual instruments. The HD600 does exhibit the 3-blob effect in comparison. Live Acoustic sets are a joy on this headphone - if the lack of thump does not bother you.
Driving - these buggers demand power. I thought the HD600 was hard to drive - guess againPowering them with a Lovely Cube with OPA627 biased into Class A by 3.7dB, and 20dB Gain set on the switches. The HD600 is easily wrangled by this amp, But the AKG is more challenging. I do believe it is the lean Mid-bass that causes me to crank the volume up-but to its credit, the K601 never, ever shows any inclination to distort-except when severely underpowered, such as when I directly plugged it into my D2 - maximum volume was translated to a polite volume in the K601, and the bass fell apart.
Bottom Line - No Power, No Go.
Comfort and Build - No sign of the Sennheiser Clamp of Death here. The automatic band is a pleasure to use, and the roomy earpads are very comfortable. A point to note -The Roomy Earpads require you to be a little careful in placing the unit over your head carefully-following Dr. Jan Meier's Advice of Low-And-Forward does enhance the Soundstage to a spacious 3D space, and affects tonality positively. On another note, the slimmer cups of the K601 (vis-a-vis the HD600) make it easier to wear in bed.
The Phones are as light as a German Sponge Cake, and unfortunately also feel just about as delicate. The HD600 feels far more Robust, well-thought out in terms of Modular Design, best of all, built to last.