Hi all.
First, I've been inspired of work that people make here, it's very helpful and informative.
I've been curious about tuning of headphones (as hobby), tried to build my own measurement rig (very low budget, but successfully). It includes noname electret mic, TS472 board and flat pinewood panels. Then I made some measurements, and K612 were among many other headphones that I had time to test.
Of course, these measurements aren't perfect at all, but I have K702 (2009) as starting point. Apart from some displacements of resonant peaks and other rig issues, general form of graph was kinda similar to graph on DIY-A-H.
But when I tried K612 (2020), they were unexpectedly different than K702. Don't know what it is, maybe some changes were made by manufacture after testing in 2015, but my sample hadn't any upper mids elevation (1.5dB max), that annoyes me at K702.
About sound, I found this K612 very comfy. Nice bass (my rig can't display it, but it's really good for most of music genres) and plane mids. Their treble peaks wasn't problem for me.
About build quality... all of high-level AKG's that I've seen were horrible after years.
1. Elastic bands loosed its tension.*
2. Transparent plastic holders from inside of headband cracked.**
3. Cable oxidation was catastrophic. Copper becomed black, green liquid tried to pollute left driver, headband steel rods and right driver.***
4. Hinge construction that sometimes tear nylon mesh of acoustic baffle on drivers.****
*Can be replaced with disassembly.
**Doesn'a affect to functionality.
***Cable of K601 (left driver and steel rods were completely damaged by oxidation products), K702, K612 (recently), two cables of K271MKII. Ironically, the cheap cable of K99 that I used for replacement was in fine condition.
****Both of K702 drivers, recently. Unbelievable engineering or production mistake (or both). I've seen marks of touches by hinge construction on K612 drivers too. Grind some plastic on hinges by file was helped.
Without it K612 model would be the great product. Fortunately, there are only few occasions like these I've saw in web. Maybe it's unique bad luck, but now I can tell so much about AKG headphones, LOL.