OK, after listening to this pair for a few hours here it is:
Quick note:
HD490 Pro is stock, while both HD599 and K612 Pro are re-cabled, internally with silver-clad copper wires and externally with Belden 46349 cable.
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Bass:
Good quantity and extends quite well to the lower octaves.
With velour pads is a bit on the slower/softer side but more abundant than with the fabric pads where it becomes leaner but faster.
Compared to the HD599 with either pads, is less pronounced but better balanced since the HD599 can get a bit muddy.
Compared to the K612 Pro with either pads, it is more pronounced and rich in the mid bass, but with AKG having a slight edge in terms of speed.
Mids:
Balanced and well defined, with maybe a little recessed character which helps to the non-fatiguing character of these headphones.
Compared to the HD599 with either pads, are more forward and dryer but without getting grainier.
Compared to the K612 Pro with either pads, are more recessed and less rich, with AKG gaining on clarity and speed.
Treble:
Soft but quite well controlled and with decent extension, although my ears are limited range so keep that in mind.
With velour pads they are obviously softer than with the fabric ones but they never get tiring. With the fabric ones they gain a bit of definition.
Compared to the HD599 with either pads , is just a bit more pronounced without being more grainy or splashy and generally are non intruding.
Compared to the K612 Pro with either pads, is slightly less extended with AKG having more clarity although the AKG can sometimes lose control becoming splashier.
Overall:
I like this headphone for its overall balanced tone and non-fatiguing character.
Its sound stage is at the level of HD599, which means definitely above average.
Imaging is also equally good and with fabric pads a bit better even.
AKG being a faster headphone, is slightly better in terms of sound stage width/depth and imaging accuracy.
But don't forget this line of open AKG cans are known for their extraordinary staging abilities, so it doesn't mean the HD490 Pro has sub-par stage.
My only criticism over the HD490 Pro is that it can be a bit slow at times.
It can feel a little grainy when things get complicated especially in the higher-mids and lower-treble regions.
I feel it loses impact, usually on percussion instrument because of that, feeling somewhat softer than both aforementioned headphones.
Of course, one would need to constantly and furiously rotate headphones over the ears to identify such differences conclusively.
And even then, they are such small differences, I doubt after 10min pass one could maintain sound memory to pinpoint the details.
But, we are compulsive and neurotic, otherwise we wouldn't be in that hobby i guess.
So my feeling is that the main culprit for lack of speed/softness of impact, is probably the dust pad.
To me, it looks a bit too restrictive over the baffles making the driver harder to breath.
Will experiment with some material i have in mind and be back with impressions, but i am pretty sure it can be improved.