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.
On good-sounding recordings, we definitely preferred the SR225i headphones because they sounded more realistic. Puente Celeste's "Nama" CD is an audiophile recording, so the voices, clarinet, accordion, acoustic guitars, bass, and percussion on this world music CD had an almost 3D presence and solidity on the Grado pair. Switching back to the HFI-680s, "Nama's" soundstage was squashed flat and the treble wasn't as clear. At this point we turned the Onkyo receiver off and started listening through the Schiit Asgard headphone amplifier, which improved the sound of both headphones, but we still preferred the Grado on better-sounding recordings, and the HFI-680 on compressed rock music.
We next auditioned the HFI-680 in our home theater as we watched the "King Kong" DVD. Kong's grunts and roars were fearsome, but we felt the HFI-680 headphones foreshortened the soundstage depth. The jungle scenes populated by buzzing insects and birds didn't sound far enough away, but when the dinosaurs stampeded, their pounding feet made the jungle ground shudder.
We're not sure how many of you would want to use such a large headphone, with its 10-foot-long cable, plugged into an iPod or Zune, but the HFI-680 sounded spectacularly good on the streets of NYC. Bass was rock solid, dynamics opened up, and the midrange clarity was well ahead of what Monster Turbine in-ear headphones ($300) can achieve. If you can handle the extra bulk, the HFI-680s won't disappoint.