To that point: one definite advantage of the HFI-680s' closed-back design is that it hushes outside noise. It's not as completely isolating as a bona-fide noise-canceling headphone, but it gets you halfway there. The isolation also works in the other direction, limiting how much of the headphones' sound will be heard by people around you.
The HFI-680s, like all of Ultrasone's headphones, feature what the company calls "ULE technology" (ultralow emission). The company claims that most headphone drivers produce low-frequency magnetic fields. Ultrasone employs MU Metal shielding (a nickel iron alloy), which--again, according to Ultrasone--reduces magnetic radiation by up to 98 percent compared with other headphones. It all sounds quite impressive, but we have no way of independently verifying it.
We listened to the HFI-680s over an Onkyo TX-SR805 receiver, a Schiit Audio Asgard headphone amplifier ($249), and an iPod. The Onkyo sounded more laid back and mellow compared with the Schiit Audio amp, so yes, the headphones' sound quality changes, depending on what it's plugged into. Our first impression of the HFI-680s' sound with the Onkyo receiver was, wow, these headphones make a lot of bass! Not only that, the bass is deeper than we've heard from similarly priced open-back Grado and Sennheiser headphones. The open-backed headphones may make less bass, but the bass definition of the Grado SR225i was better than the Ultrasone's. The HFI-680's treble has a sweet, refined character, so it's easy to listen to these headphones for hours on end.
The HFI-680s sounded surprisingly good on heavily compressed and harsh recordings, like the new Arcade Fire CD, "The Suburbs." That recording's flaws were hard to ignore over the Grado SR225i headphones, which seemed to highlight the most irritating aspects of the disc's sound mix. The HFI-680s' softer, less pronounced treble and bassier bottom-end took the edge off.