Performance

The Ultrasone HFI-680 is aimed at people wanting a solid performing headphone based on a closed back design. With a price of around $230 it costs just as much as my AKG K701 or Sennheiser HD580s with various mods. The differences between open and closed back designs are vast, but since they reside in the same price tier the open headphones will act as a good marker for whether the HFI-680 perform well, price considered. To test just how power hungry these headphones are I tested them with a lot of different sources and amps.
Like with any other decent set of headphones a good amount of burn-in is required for them to sound their best. Straight out the box these headphones were extremely sibilant and made the cymbals sound very painful. After 150 hours of playtime the high end was dimmed a bit which made them much easier on the ears.

The midrange is reasonably detailed, however, not quite on par with that of my open headphones. It is good just nothing special. That being said they are far from being the headphones with the worst mids around.

To start off with I hooked them up to my Cowon D2 DAP to see if they could be driven well by it. The sound quality was only so and so and not that lively. The addition of a RSA Tomahawk to the chain really improved the sound quality a lot, more precise and deeper bass with a slightly warmer and forward midrange. I am pretty pleased with the sound, but you should definitely use at least a portable headphone amplifier with them since the reward in terms of sound quality is substantial. Unlike my other high end headphones the Ultrasone HFI-680s can be run from my portable setup with a decent outcome. Both the AKG K701s and Sennheiser HD580s are far too power hungry to be driven by a small headphone amp like the Tomahawk.
Just for the fun of it I tested them with my Travagan's Colors mini booster amp and here too the sound was quite good, a bit less bass and more emphasized highs, but the general sound quality was very pleasing.

The HFI-680 headphones are not analytical in fact they have a very fun sound signature, with a smiley face frequency response. The bass is big and powerful when driven by a decent headphone amplifier, the gracious amount of bass suits most types of electronic music, rock, and pop well. It is not overpowering and does not ruin the overall presentation of the music unlike other bass heavy cans. To begin with I was a bit afraid that these headphones would have a closed in sound with a way overpowered bass, but that did not turn out to be the case. These headphones have a very good sound stage and do not sound boxed in, that being said they do not sound like open back headphones either so do not expect miracles. The sound stage capabilities of the HFI-680s is quite good but it is more a left or right type of presentation than that of my AKG K701s. The Ultrasones do not give you the same perception of depth through their presentation, it is not a big thing, but something that adds a bit of realism to my ears, some will undoubtedly argue to other way around and call the AKG headphones uninvolving or even dull.

Because the Ultrasone HFI-680 cans are relatively easy to drive you can get them to sound their best, or close to, with a modest headphone amplifier like the Travagan's Green or Octavart "The One" Amp and DAC. Of course they do benefit from proper amping, I found the sound quality to be outstanding with my Head-Direct EF1 amp hooked up to my Marantz CD6002 player.

The HFI-680s strong side is the bass, it is plentiful and well defined. The mids lack a bit of texture compared to that of my AKG and Sennheiser headphones. Also the highs are a bit less detailed on the HFI-680. For electronic music and trip-hop the bass kick of the Ultrasones is very pleasing, but for rock and other vocal driven music I still prefer the AKG K701s and Sennheiser HD580s being driven by a good amp due to a more detailed midrange. For use on the move the HFI-680 are by far the best because they can be driven well by a portable amplifier and isolates you and your music, from your surroundings and vice versa.

Durability

These headphones are extremely durable compared to pretty much anything else in the same price tier. Everything about the Ultrasone HFI-680s just seems well thought out in terms of durability. I have been using them constantly for the last week and have not noticed any real flaws in the design. Of course you could scrape the cups but that is up to you and how well you treat your cans.