Look, we'll just come right out and say it -- it's awesome to sashay around and hear things right in your ears without disturbing anyone else. Maybe even life-changing. And these AKG's sound good -- very close to AKG's wired on-ear options, and certainly worlds beyond the Radio Shack wireless headphones we've experienced in our past lives. But to be honest, we don't really particularly like the on-ear design in general. We much prefer in-ear (see Etymotic's line) or over-ear (see Sony's studio standard MDR-7506). These things feel a little like over-ears that shrunk in the wash.
For as fantastic as the idea of wireless is, it's not awesome when the wireless signal cuts out about 15 or 20 paces from the source. The transmitter feels very directional: while the signal seems strong 30 feet east of the transmitter, it might start breaking up just ten feet west of it, without any real interference-inducing materials in the way. While we can certainly picture distinct uses for the K 840 KL's, possibly in the kitchen or bedroom -- we just wish they worked a lot better. It kind of feels like we might as well have just used a really long cord. If you're just looking to go cordless from your device in your pocket to you ears, though, these would do the job nicely.
The Bluetooth sibling of the K 840 KL is the K 830 BT, priced at $249. It's very similar in layout, but includes transport controls (play / pause, fast foward, etc), a "call" button, and an integrated microphone on the left ear casing. We found reception to be fairly strong within the advertised ten meters in all directions from our laptop when using the phones as our primary sound device. But, as we've experienced with Bluetooth headphones in all corners of the market, the audio quality is disappointing. The K 830 BTs suffer from a bad sort of "crispiness" at the high end of the spectrum: female vocals seem to suffer the most. Pairing a cellphone with the K 830 BT is a charm. Audio quality is high in both directions. But we can't really see why we'd choose these over a smaller Jawbone style unit just for calls. We were also disappointed with the build quality. The silvery paint on the buttons was wearing away with a fairly small amount of use, and the materials used hardly feel premium.
Wireless headphones are like all other wireless products: they're like the wired version, but not as good. We were hoping that AKG's new premium offerings would prove render this ancient paradigm obsolete, but they didn't.