Pros: General sound quality, flat frequency response, open soundstage, price
Cons: Headband design, sweat-inducing pads
Construction and comfort
Clamping force is rather tight, but not uncomfortably so. It's possible to adjust the force simply by carefully bending the two metal bands keeping other parts together. The metal headband can make annoying (and very audible) sound when it interacts with the plastic part of the band and starts to vibrate. This might not be a problem for persons that have smaller head and it should be easy to dampen these vibrations with some easy modifications.
Pads pleather material feels slightly softer and thinner than the ones used in m50 but the general feel is just as good. They will induce some swetting just like any other similar pads. It is possible to change the pads with AKG K-240 pads just like HD668B pads were - I haven't tested their impact to the sound quality yet.
Isolation is pretty much the same as ATH-m50 provides, quite good.
The sound is clear, well balanced and transparent. Detail retrieval is excellent. All in all, I couldn't find any obvious flaws. As the frequency response is near-flat these phones shouldn't get too tiresome either.
Soundstage of 662F is *a lot* wider than ATH-m50. Players seem more closer to the listener with ATH m50 - soundstage is small but well defined. 662F is very spacious in contrast. Maybe even slightly more spacious than 668B! It's hard to believe these cans are closed.
Direct comparision to ATH-m50: 662F has less bass rumble but has similar extension. Bass is also tighter. 662F sounds little thinner due to having less lower mid emphasis. It also has brigher highs.
Direct comparision to 668B is harder as the cans have pretty similar sound signature. General signature in both is very neutral. 668B seems to have slightly more mid emphasis making 662F sound more "open" but "hollow" in contrast. Without direct comparision its hard to tell the difference though.
Currently HD-662F are priced 33€ (~$48) at Thomann.de. I goes without saying that they are huge bargain and similar value as 668B.