Summary: 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.
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.
Summary: I came to this headphone after 10 months with a Philips X1. Although the X1 is a good headphone, towards the end of my time with it, I become increasingly dissatisfied with its sound signature.
I stopped listening to music regularly and for a long time I didn't realize that the X1 was to blame!
Pros: Well-balanced, natural sound, analytical yet musical, bass deep enough to satisfy and tight. Comfortable.
Cons: Slightly dry sound, needs a strong headphone amplifier.
I got my pair on sale for $150 USD. I have had a few lower end AKG headphones and have never been disappointed with the value and sound signature they offered. It's been a long time coming but I was finally able to get a higher end AKG in the K612 Pro.
Pros: Good value, balanced, accurate. Audiophile grade sound for a bargain.
Cons: Larger heads might have fit and/or comfort issues. Lacks bass impact compared to higher end competitors.
Summary: this is my first "serious" pair of headphones So the review cant be very full, but I try to give the possible useful information. I was looking for a pair of headphones with a neutral sound but with a good bass impact and suitable for gaming, and i am very satisfied with them.
Pros: neutral but good impact in the bass, tights bass, price,great for gaming, design.
Summary: Updated review: 5/28/15
Build Quality: Great build quality. Overall solid professional looking open back headphone. Cable is strong and thick, non detachable cable isn’t/wasn't a big deal breaker for me, still compared to 7xx series, 612 uses lesser quality materials (plastic, leather headband)...
Summary: I have owned several other AKGs, including the K240s, the K702 65th Anniversaries, the and Q701 Quincy Jones Signatures (basically the same thing as an updated K701/702). The K612s sound-wise lie somewhere between the K240s and the K7-series, while having a unique sound signature of their own.
Pros: Roomy ear cups, smooth, linear, natural sound, non-fatiguing highs, well behaved bass, AKG mids, not hard to amplify
Cons: initial ear pad comfort, picky with DACs, hard wired cable (good quality though), not for bassheads, synergy with systems