Pros: Detailed, transparent, smooth and linear sound, revealing, very natural sound, precise imaging, good sounstage, non-fatiguing, and comfortable.
Cons: A bit finicky to amp, needs a good amount of power, and pads take a few weeks to break in.
This is a brief review of the K612 Pro, to my ears it's my favorite and the best headphone under $200. I'm using a Hifiman EF2A as an amp and an ASUS Xonar DX as a DAC.
Build Quality: The K612 is a well-made headphone and feels solid in my hands. Only gripe is the cable isn't detachable but it's not really something I'm worried about as the cable quality is good and I take good care of my headphones.
Comfort: This headphone was comfortable from the beginning to me but the stiff pads did bother me after a bit. A few weeks of use and the pads soften up and become much more comfortable. I have no complaints about the leather headband, there are no bumps and is plenty big enough to distribute weight well.
Amplification: This headphone must be amped and have good synergy for it to reach it's full potential. If it's underamped it may clip a bit in the sound and sounds lifeless, dull, and flat sounding. People buying this headphone need to go in knowing it needs to be amped. It seems to respond the best to warmer sounding systems.
Soundstage: I personally find the soundstage on the large side on this headphone, while it's not huge, it's not really what I call medium sized either, just simply large. The soundstage has good width and the depth and height aren't bad either.
Imaging: The imaging of this headphone is very accurate and precise, this headphone excels imaging. You have a very good idea where the instruments are located and there is good separation.
Bass: Bass when the headphone has good synergy is very textured and detailed, extends unusually deep for an open headphone and has a large and powerful presence with plenty of impact.
Midrange: The midrange on this headphone is what some may consider forward, it's delicate, precise, has good dynamics, and just sounds very natural and organic. It's just beautiful sounding.
Treble: This headphone is rather sensitive in terms of it's treble response from system to system. It is detailed and revealing, so flaws of the recording aren't hidden. It's quite accurate but can have sibilance issues with some recordings and systems and some systems the 2-4khz region may be a bit too present which can lead the headphone to sound a bit hard and dry on some systems. It has a good and accurate treble, it is slightly rolled off at the very upper registers and doesn't have a lot of air like some other open-backs but it's not lacking air by any means, it's just not an airy headphone.
Conclusion: The AKG K612 Pro is an excellent value for what you pay and is an excellent headphone for people seeking a neutral and linear response. It's among one of the most neutral I have heard and is a real treat if you are willing to work with it to bring out it's potential. The headphone is rather sensitive and is very transparent for it's price. It's a headphone that needs a good system behind it that synergies well with it.