Pros: VERY deep bass, much improved comfort over K518, clear sparkly but not harsh highs, lush and moderately detailed mids with good definition
Cons: Still a bit uncomfortable for the long haul, Soundstage compressed at times
K518 is no doubt a legend - killer sound and decent isolation for a portable cans, together with the most badass clamping force ever.
K67 is one full class above in sound, and maybe two in comfort. Funny though, the K618/619 siblings exerts a MUCH stronger clamping force on the skull than the K67, despite the seemingly similar design.
Bass: As deep as you can hear, and as deep as you can feel as well. This is definitely THE cans to produce clubsound in terms of bass, but with much better everything else. The K618/619 can reach just as deep but while K67 emphasize sub-bass while keeping mid-bass less prominent, the K618/619 exhibits a tamer sub-bass but with more mid-bass emphasize.
Mids: Lush and clear. The K618/619 siblings sound a bit more open in mids, but the K67 sounds quite a bit more warm, natural, and easy to the ears.
Highs: Both the K67 and the K618/619 produce some twinkles and shimmers in the highs, but they do roll off gradually. You won't find the last bit of high pitch overtones in these, but in exchange you won't find them screechy or harsh.
Conclusion: Competition is stiff for the portable headphones market these days - Senn got two killer cans in the Amperior and the Momentum in their arsenal, each got its own appeals. AKG, instead of pulling out completely new concepts like the Momentum, chose to keep its winning formula from the K518 days and improve on it, while keeping the price VERY budget friendly, contrary to Senn's take. To be completely honest I'd take the K67's sound over the Momentum any day - I've never liked its overly warm tones and all-engulfing, but not deep enough bass. The amperior is superior in most aspects BUT for the capability to produce deep bass from a non-amped source. The K67 is a revelation - at its price point it's hard to beat. At twice (or more than twice) its price point, where you'd find the Momentum and the beats and the B&Ws, it still holds its own very admirably. Time will tell whether it makes it to the legendary status of the K518s.