The AKG K391-NC are a good set of in-ear buds that sound great and isolate the listener really well. Their outstanding noise cancellation and leakage performance are recommended for noisy commutes. The sound profile works well for most music genres, but their in-ear design might not be for everyone.
Above-average as everyday headphones. They're compact and have a good sound. They can also handle loud environments well, thanks to their great noise cancelling. This makes them versatile enough to use in most situations.
Average for critical listening. Above-average sound quality but the small in-ear buds aren't optimized to produce a good soundstage, which isn't ideal for pure critical listening.
The AKG K391-NC have a great isolation performance, which makes them easy to recommend to anyone who travels a lot or has very noisy daily commutes. They'll reduce a significant amount of ambient noise, and they're easy to carry around.
Above-average for sports. The AKG K391-NC will easily fit into most pockets, and they're stable enough to run with. As long as are comfortable with the in-ear fit.
Above-average for studio use. Leakage is barely audible and won't disturb anyone around you even at higher volumes.
Above-average for home theater use. The AKG K391-NC have no latency but they also have a short audio cable. They have the range to watch TV from your couch unless you have an extension cord for the audio cable.
The AKG K391-NC's design is very simple. They look good but are a little unexciting. The control module has a brushed metal finish that's also noticeable on the earbuds. The cable is black and the rest of the accents are a deep gray, matching the AKG branding.
The AKG K391-NC are as comfortable as you would expect in-ear earbuds to be. The sleeves come in three sizes, so unless your ear canals are unusually narrow or wide, you should be able to get a comfortable fit.
The buttons are in the right place, and the Control Module is small enough to fit any pocket. The in-line remote has a mic and call/music control. No volume up/down.
The AKG K391-NC are decently stable headphones. Their in-ear design fits deeply into the ear canal making it hard for them to fall out during moderate physical activity. However, they won't be sufficiently stable for high-intensity exercises, and the control module and long cable can easily get hooked on something and pull the headphone out of your ears.
The AKG 491 are highly portable headphones. The in-ear design is compact and doesn't take too much space which makes it easy to transport in a bag or purse. The control module adds a little more bulk than basic passive in-ear headphones but doesn't reduce their portability as they will still comfortably fit into most pants or jacket pockets.
The AKG K391-NC come with a soft fabric pouch that will protect the headphones from scratches and doesn't add much bulk. However, the soft fabric isn't waterproof and won't protect the headphones from hard falls or water damage.
Except for the in-line remote, which feels a bit flimsy, the rest of the headphone has an even and decent build quality.
The AKG K391-NC come with three cables (in addition to the non-detachable part): one USB for charging, and two 1/8" TRRS-TRRS with different plug colors.
They have excellent noise isolation, especially at higher frequencies. Passive noise isolation is very good down to 500Hz. Below that, ANC kicks in, which maintains excellent isolation performance. The only point where they seem to struggle is around 400Hz.
Very good leakage performance. The AKG K391-NC leak more around 4kHz, which means higher notes will escape.
The AKG 491-NC have a long battery life but take considerably longer than average to charge. This may be because they use an older mini-USB charging port. Unfortunately, this means you have to charge your headphones overnight which is not always practical. On the upside, they deliver above a day worth's of battery life, and you can use them while charging so they are decent headphones to use at the office or if you have access to a power source while on the bus, train, or plane.