Pros: Neutral, slightly warm, versatile, comfort, stock cables & connector
Cons: Can be slightly boring, feels plastic
This will be a somewhat compact review and comparison to the HiFiMAN HE-400, they're roughly the same price around here. Starting with some pictures.
While I don't consider accessories that important I guess I should mention them. K712 comes with a straight and a coiled cable, 6,3mm adapter, and a carrying pouch.
HE-400 comes with the stock cable, 6,3mm adapter and a carrying pouch.
The comfort of the K712 is good. The headphones weight is 235g which is very light considering their size. The self adjusting headband works well and the earpads feel nice on the skin. I don't think the fit is too loose despite having a small head. During the first hours of use there is some pressure building up on the head from the headband, but that disappears after a while when it adapts to your skull.
Compared to the HE-400 they're obviously much lighter, HE-400 weights 440 grams which is almost double up. The headband on the HE-400 is very well designed though, and I don't have any comfort problems with them. So apart from bulkiness HE-400 is just as comfortable. Both stock earpads are fine too although I know some people don't like the HE-400 pleather pads, if that's the case then you probably like the K712 pads more which is soft memory foam.
While the build quality in general is good, the K712 does feel a bit plastic considering the price. Especially the bows that connect the cups feel very fragile. The headband and cups themselves are good though, and the mini XLR connector for the cable works well. The stock cables that come with it are good.
In comparison the HE-400 is built like a tank, but the coaxial RF connectors are bad and the stock cable is extremely bulky.
Sources: CA Dacmagic Plus DAC, Lake People G109-S headphone amp
Files: My own FLAC library and some Spotify Premium streaming
First I'll focus on the K712 and after that a comparison.
As a whole, the sound provided by the K712 is impressive. I never really find it to offend or do something bad. It does expose bad recordings though, no way around that.
The bass is very well controlled. With music where slam in the bass is not intended you get none, zero. However, try turning up some Infected Mushroom or Ludacris and you will be treated with some serious rumbling or impact, depending on the song (for an open headphone). The presentation is very convincing on "Ludacris - How Low", I can dig out all the low frequency details in the background and the rapping is in perfect balance with the beat. With "Koan's - Selena's Song" it's very easy close your eyes and be carried away by the firm bassline backed up by the deeper rumbling and the ambient sounds, I imagine this is very close to how the artist wanted it to be.
Varieties of rock is probably their strongest point for me though. The balanced yet slightly warm presentation together with the rather wide soundstage is a real winner. In "Dayshell - Avatar" I can separate instruments very well and get a sense of how the song was recorded, but it doesn't get too analytical, it's very easy to still simply "enjoy the ride".
With metal you do run into this problem where a large portion of the music is rather badly recorded and mastered, and the K712 does expose it quite effectively. When it's good it's really good though, as an example my In Flames Clayman and Reroute To Remain rips sound brilliant. I do occasionally wish the drums had a bit more "kick" in them though, something the HE-400 has spoiled me in.
The K712 does vocals so well that I can really appreciate music like Celine Dion's despite I don't normally have any interest in it. Material like the old Nightwish with Tarja Turunen sounds fantastic.
Regarding the treble there isn't much to point out. It's smooth, it's good. No fatigue, no emphasized sibilance, amount is just right.
Not much to add. But yeah the sound is great. To counter all this praise, I do find the sound gets a bit "too secure" sometimes. It's like, you listen, and you know it sounds great, yet it's a bit boring at the same time. But that's probably inevitable with something that performs so even. I think a 4/5 star rating is fair, could maybe be 4½, but it loses a bit on the build quality, and on price. I haven't actually compared it to the older models, but if there really are changes made that qualify a ~250€ rise in price I think AKG should make that more clear. On the other hand, they clearly compete with the HE-400 that are priced similarly, so...
Comparison to the HE-400
When you compare these two headphones, the weird V-shaped frequency response of the HE-400 is very noticeable. The HE-400 has a great bass presentation in my opinion, and the texture on drums is brilliant. On the same time, this combined with the rather small soundstage gives you this very intimate and warm sound that can be a bit too much. Vocals also sound a bit strange on the HE-400 when compared to the K712, this is probably related to the dip between 5-6 KHz. HE-400 also has slightly more emphasis on treble, especially sparkling sounds like cymbals, sometimes this works out great, sometimes less so. HE-400 definitely has a higher fun factor, so this does boil down to taste. I would still say though that the rock solid performance of the K712 makes it better overall. There is slightly more sibilance on the HE-400.
AKG K712: Is more neutral, has a larger soundstage, is an more even performer.
HE-400: More bass impact, slightly more treble, does some material very well while falling short on other, more forgiving with bad recordings