Great review, congrats ! I have one K612 and i love him !

Good review. Straight-forward and understandable. 

I find your verdict to very interesting because I own both AKGs and I owned the AD900X, I sold them after I got the K612 because I was never using them anymore, though they are a lot more comfortable than either AKG.
Anyway, the only advantage to the K612 is a natural sounding soundstage, however it sacrifices width.The Q701 is more detailed and it does not have that slight grain to the sound of the K612. I don’t find the bass fuller on the K612, it’s quite similar from that point of view but the Q701 can produce the impact that the K612 lacks, though as you said the highs can be a bit fatiguing for some.
In order to improve on the above mentioned I feed the Q701 a slightly warm sound, it brings out more bass, it produces decent impact, mids still sound good, the highs are tamed, they still have quite a bit more detail than K612 with a neutral sound source and they still retain the wide soundstage.

Good review. I have both headphones myself, though the K612 is being borrowed currently. I do think the HD 600 is more refined and natural sounding overall. I do find the K612 has a bit of grain in the upper mids, the HD 600 does as well to a lesser degree. But despite this, some odd reason the K612 is less fatiguing to my ears than the HD 600.

Thanks for the feedback guys.

Good review Rigo.  The K612 Pro is an easy headphone to recommend.  Very good all-rounder.  I really enjoyed mine when had it.

Great review indeed.
 
Have you ever tested Philips Fidelio X2? I think both of them should be a very match rival

No I have not, but the X2 is double the current price of the K612. It would be in the league of the HD 600 and DT 880. Definately another contender though.

@rigodeni , @dividead  Can you please explain more on AD900X vs K612? I did own the K601 and the AD1000 for some time, and I preferred the K601 much more than the ATH. I'm pretty sure both the K612 and AD900X will have huge soundstages and good mids, but what are the main differences between the 2? 
 
I'm currently having a O2 and a LD I+

Take my comments on the AD900X with a grain of salt since I have not owned or heard them myself. I am simply repeating what I have read. The AD900X is brighter, with a more airy and expansive soundstage. The K612 is more neutral, with fuller bass response. Comfort wise the AD900X is a love it or hate it thing with the winged headband design. I tried other models with this same design and I can't stand it. The K612 has a more versatile sound signature that will please on more genres, and is a safer bet fit and comfort wise too. Another one to compare with the K612 is the K7xx. I got to hear it last weekend during a head-fi meet. It's only available on massdrop from time to time. But at $200 it has a better build with a bit fuller bass and more comfortable memory foam ear pads.

Unless I'm mistaken, you've got some Beyers in the background (DT 770 I think?), how would you compare them to the k612? (I know they have quite different sound signatures, I just want to hear from someone who has heard both). Thanks. 

I did a full review of the 770 here. Below I quote myself on a previous post comparing it with the 612.
 
"I actually love the DT 770 just as much, but they are different enough to justify owning both. The 770 is aggressive, particularly in the low end, while the 612 is more balanced and refined. I find myself going to the 770 more so due to being closed-back while being just as comfortable."
 
In terms of durability and build quality, the 770 is in a whole other league. I think the 770 is a safer recommendation for most people because of it's closed-back design. But it's also $50-$100 more. Note that it has a completely different sound signature though, much more "V" shaped response. The 612 is more neutral and resolves details better making it superior for monitoring applications. They are both excellent, but so different, which is why I own and plan to keep both.