So, I just got the K712 and had the chance to try it only briefly.
Impressions below, but probably have to apologize in advance if I didn't seem to explain as well or accurately as I should (new to all of this)
First thing I immediately noticed about this headphones is on the soundstage. I think the first thing I said when I had them on was "oh god". I know AKG has been said to have really good large soundstage, but I didn't expect it to be THAT big. It's almost way too overwhelming for me, actually. I'm not too sure I'll get used to this.
I said previously how worried I am about the bass compared to the Fidelio X1, and therefore losing the "fun" factor, but I'm actually quite happy with the amount of bass in this. They're definitely there, but not heavy by any means, and are quite balanced and elevated. It's almost good enough for me when listening to EDM or trance music. MUCH better fun sounding than the safe HD558 for sure. Having said, I don't see myself regretting not getting the X1 on the bass department, but still do in a few other regards (who knows, I might be wrong)
Comfort wise, this is perfect and is probably as good as I can see it go. Headband is pretty much perfect. Only complain I have is how the earpads are inserting quite some pressure, especially at the top part of the head (I assume this will be rectified with repeated use, hopefully?)
I've also had the chance to try them with an E10 and E12 (friend's). I saw a lot of posts of how the E10 is bad match with the K712, but man, I think I might have to disagree with that. The E10 made things brighter and improved the bass, and might just be my current go-to for music and movies for now.
Overall, first impressions with the sound quality is quite good. AKG's sound is definitely different to what I am used to. I really enjoy the clarity and the "realistic" feeling out of these though. Another additional con when hearing this was how I felt some of the sounds were kind of "staying back" (not really sure how to say this). Having said that, I'm not too sure if whether these are a keeper, I probably will have to listen to these further before deciding to keep these.
I'm also almost thinking whether closed headphones are actually better for me at the moment, even for overall and gaming use. I had the chance to demo the Momentum on a nearby store, and I loved what I heard out of these, making me believe the overall nature of closed headphones might be the reason behind it.