This is essentially a K601 review, however I will reference it to my AD900, as I am currently looking to find a “better” AD900. My review, my rules.
Design
First off these two could be brothers (or sisters) from a design point of view. They both are large open circumaural headphones with the variation of the two manufacturers wire support wings/headband combo. Both are very comfortable. The AKG feels slightly heavier, albeit it isn’t. I think this is because in clamps against the side of the head whereas the Audio Technica does not (it floats), well at least not on my head. Having said it clamps, not like a closed fon that is trying to sandwich your brain, more simply clamping for a secure fit. You can head-bang wearing the AKGs, whereas the ATH would probably fly across the room. Both fons are big with very large circumferences, although they are almost identical in size the K601’s feel smaller as they don’t come down as far as my lower jaw, as do the AD900s. All up the K601s are very comfortable and I can wear them all day without a whimper. The build quality has been questioned in some reviews of the AKG’s, being criticized for being too plastic and feeling fragile, I do not agree with this perception. They are primarily plastic, however it feels and looks like quality to me, and there is no impression of fragility to my mind. Finish is exceptional.
Ok so they are comfy and I like the look and feel of them, how do they sound?
My opinions are based on my system- Sony DAP mostly with 320 bit rate tracks with a couple of more compressed files thrown in for comparison, running through a Meier Corda Move with 12V power supply using high gain. Volume adjusted up to compensate for the AKGs less efficient 120 ohms.
Sound
The K601 are somewhat midrange centric but are fairly well balanced, even sounding, they don't exhibit any one aspect at the expense of the balance of the audio spectrum.
Everything seems part of a whole. Frequency response seems fairly flat. They are coloured but lean toward neutral, probably more neutral than the AD900.
The K601 is a midrange specialist (so to is the AD900) vocals are especially noteworthy, particularly male vocals (whereas I prefer female vocals projected by the AD900, although it is a close call). Sound is very open with natural timbre and dynamics. Bass is controlled and tight.
K601s are detailed without sounding analytical. The K601 is a bit less energetic than the AD900, slightly more laid back at times, but is smooth and sweet like the Audio Technica and also has an ability to keep instruments and voices separate in complex music. The K601s have a great soundstage that comes close to providing that “speaker like” feeling which is a neat trick for any headphone.
Although I wouldn’t describe them as so laid back as to be “boring” or “forgettable” they are not “exciting” or “fun” headphones either. They are very comfortable, easy listening, non-fatiguing headphones, and ideal for acoustic music with vocal emphasis; folk and ballad type music. Guitar strumming sounds sweet on these as well. They may also make good classical fons, however that is not my genre so I shouldn’t speculate. They are not ideal for rock or electronica, lacking the dynamics and fun factor IMO.
Treble
The treble isn't as extended as other higher end headphones, but they still manage to reproduce the upper range with a clean, clear sound, cymbals still sound great. Though the highs remain quite clear and simple sounding without being overly harsh, they are slightly recessed. The highs and treble in general are delivered well enough without distinction, with the bonus of being devoid of sharp, grainy scratchiness in the mid and upper treble frequencies, most of the time.
Midrange
The midrange is all one could ask for - vocals, both male and female come across as real and instrumentals are handled with style, preserving the natural timbre in instruments.
The AD900 has a sweet, sweet midrange, detailed and silky and there aren't any dips or peaks. Vocals are excellent, clear, precise, and realistic, with a “live” sound to them. The AKG K601 has similar sweet midrange properties, however with a more studio sound to my ears. Voices and the midrange in general are just fabulous with excellent detail retrieval.
Bass
Bass is good, deep enough and solid and integrates smoothly with those wonderful mids. Similar to the AD900 the lows do extend, but don’t give the impression of power and depth, you do not get the rumble and weight of the lowest bass. The lower end isn't a very powerful or impactful part of the K601s sound. However, mid bass is tight, with good impact, speed and accuracy, not boomy in any way.
Overall
The K601 is a great headphone, well controlled, smooth, and very evenly balanced with a nice sense of transparency and spaciousness in the presentation, with super sweet and detailed mids. Great for easy listening all day to your favourite acoustic tunes, detailed, clear, precise and with a lovely sound stage. The K601 is not an exciting, dynamic, fun headphone, with gradoish attack. Although the K601 has many similarities with the AD900, and I am sure some may prefer it to the open ATH fon, to my ears, with my gear, playing my tunes, it is not a better AD900.
AD2000 or SA5000 are still on my horizon.