Pros: Well-balanced, natural sound, analytical yet musical, bass deep enough to satisfy and tight. Comfortable.
Cons: Slightly dry sound, needs a strong headphone amplifier.
I came to this headphone after 10 months with a Philips X1. Although the X1 is a good headphone, towards the end of my time with it, I become increasingly dissatisfied with its sound signature.
I stopped listening to music regularly and for a long time I didn't realize that the X1 was to blame! Although every time I used it I would be impressed by its abilities, the sound was ultimately just too bass orientated for me; and also the treble tended towards a glassy tone in some music. So after some research, in steps the K612 Pro. Now this headphone is quite different. Less bass but better proportioned, the sound is more natural with a pleasing delicacy to the sound, particularly the treble, that although a little dry, is open, articulate, never harsh or sibilant and only very slightly rolled off at the top. The mid-band is detailed, expressive and free of any nasty colourations, however the upper mid-band does have a tendency towards the already mentioned dryness of tone, but that's really a minor quibble.
The headphone images very well and keeps various musical strands from merging into each other even in busy,complex mixes. Sound-staging is good but not a particularly strong point of this headphone, it keeps everything at a medium distance; not very deep or wide. However that suits me fine as I find headphones that are very adept at sound-staging somewhat artificial in their representation of the music, not really true to life at all.
The bass on these headphones is just right for me; tight, fairly punchy and ample amounts when called for - okay, perhaps a smidgen more and it would be perfect, but considering this is a £100 ($160) audiophile (or audio enthusiasts, if you prefer) headphone, I'm generally happy with it. Anyway, bass quality is more important to me than bass quantity and the K612 Pro delivers on that front.
In respect of amplification; you'll need a fairly powerful, preferably dedicated, headphone amp. With low power portable devices and laptop headphone outputs you will not get sufficient sound quality or volume. So if you don't have a suitable amp, you should factor in the cost of one if you decide to purchase the K612 Pro.
In respect of comfort; there are no issues at all, not for me anyway. The headphone is very light and the clamping force of the earpieces is fairly gentle, the leather headband sits gently on the head too, and the cushions which are soft and pliant don't cause (me) any irritation.
So summing up, I heartily recommend this headphone. It performs well in all areas and shines in some, such as neutrality and resolution. I doubt you'll find a better headphone in its price range.