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
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.
Build: Made in Austria, and very close to the K7-series, but not exactly. They feel high quality, but not quite as exquisite. The plastic on the ear cups is of a slightly lesser quality compared to the K7s, but is still good quality nonetheless. The headband is a stitched genuine leather band just like the K7s. The cable is 10 feet and hard wired, so not detachable via a mini XLR jack like on the K240s and K702/Q701/K712, but is good quality. It terminates to a 3.5 mm jack with an included 6.3 mm screw on adapter.
Comfort: Keep in mind that these are not fully broken in. The K612 is right up there with the the K7s, but WITHOUT the angled ear pads and headband bumps of the K701/K702/Q701, and a harder padding than the memory foam on the K702 Annies and K712. The pads are fairly hard against the head due to the firm padding and the clamping, so will be a bit less comfortable at first. However, when they start to conform to the shape of your head, they become much more comfortable. The pads are deep enough for most people's ears, however I have a point on my left ear, and recently I found that point touches the driver covering, causing irritation. To remedy this, I took off the left pad and removed the foam ring insert. This supposedly changes the sound ever so slightly but I don't really notice. Since removing that foam insert I find the left ear pad much better. As far as I know, the K612 pads are interchangeable with the K7-series ear pads.
Lows: The K612s lie somewhere between the K702s and the K240s, with having more bass than the K7s, but less than the K240s. Both the K240s and K702s have a sub bass roll off, with the K702s having a lean bass overall, and the K240s with a strong upper bass and mid bass. The K612 have a thickish sound to them, but with the bass being surprisingly linear across the board and without any particular hyping of the sub bass, mid bass or upper bass. As a result I find the K612 has a very clean and accurate sound to the bass. Now bassheads might not like this, because for tracks that are mastered to require a boosted bass, the K612s will sound untextured and the bass will end up sounding artificial. However, if the track is mastered to not require a colored bassline, or is natural bass from recorded instruments, the K612 does exceptionally well. Like most other AKGs it likes recorded bass a lot, but unlike the K7s, it still does well with synthetic bass if it is mastered right. The K612s are good headphones for quicker, more transient bass like with progressive trance and liquid funk. The bass right out of the box will not sound right, and will have a dip in the mid bass, so burn in is a must to bring out the bass potential. The timbre of the bass has a sort of acoustic, instrumental or almost "wooden drum" sound to it, which gives it superb transparency for recorded music. if you listen to powerful piano playing like Alicia Keys, the K612 does an excellent portraying a sense of sound of low notes she plays. If you listen to Arabic pop or Bollywood, the K612 does a superb job representing tambourines and tables. Coming from a Sennheiser HD598 or in some cases an HD600, you will find that the K612 doesn't have that speaker-like, slightly synthetic sound to it like they have. Its bass has a very "instrumental" sound to it.
Mids: In one word: linear. They are linear across the board, without being glarey in the upper mid range like the Q701s and K702s are. Because of this, they are very easy to listen to. There is a smooth mid range without being dipped to sound tolerable, like on Beyerdynamic DT990s. I have absolutely no complaints with the K612 mids. They are pretty much perfect for me in this regard. They have that characteristic AKG house sound for musicality, so you aren't missing anything there. The mids really don't make any compromises to be easy to listen to. They aren't rolled off, they aren't dipped, and they aren't veiled like with some Sennheisers.
Treble: These headphones have no particular spike in the treble. It's quite linear across the board, with roll off happening towards the 8-10,000 Hz range. You have to play REALLY badly mastered music for these headphones to sound sibilant. The treble is there, but it isn't forward, harsh or glarey. The K612s are't as dark as the K702 Annies, but are definitely are more forgiving than the K240s, which I find are rolled off in the lower treble but abruptly have a HUGE spike at the sibilance range that makes them piercingly painful with badly mastered music lest I EQ the treble down.
Soundstage: The K612 has improvements to the bass, but there will be inevitable trade offs made. One of these to a minor aspect is the soundstage. The K612s just cannot compete with the soundstage on the K701, K702 and Q701. They do however have a more open soundstage than the K240s. Despite not having as wide as soundstage as the leaner K7s, I do find the K612 soundstage more natural due to not having the tendency to pan extremely left and right with mono recordings, causing the center stage to go dead like the K701/K702/Q701s do.
Amping: I run the K612s of my Maverick Audio A1 hybrid tube and solid state amplifier and Schiit Audio Modi DAC. I am more than happy with the system, with the K612s taking a liking to the Modi. The K612s also like the FiiO E07K DAC and battery powered amplifier. They don't need as much current as the K7s, and due to being 120 ohms, they can be run off an OTL amp like the Schiit Valhalla. The K612s like the K7s are picky with DACs more than amplifiers, so if your DAC is distorting from not being able to handle the equalizer on your computer, the headphones will make this horrendously obvious. So make sure your DAC is up to snuff. The internal DAC on Macs just won't cut it.