I'm new to head-fi and actually joined because I saw your post.
I have the AKG K495 NC headphones as well as Senn 650, 598 and PX100-II.
Where I live (Sweden) the HD650 cost exactly the same as the K495 NC.
To put it bluntly: The AKG K495 NC are in no way worth the price if you are after sound quality.
I don't know if an open headphone like HD650 are best for you, but I am absolutely convinced that there is a headphone in this price range that sounds much much better than the AKG K 495 NC, unless you are specifically searching for an NC headphone,
With that said, I do like the K495NC a lot!
Let me explain further.
Just a heads up: This is not supposed to be a proper review. It's my experience with the headphones in a specific setting. I am not qualified to compare these headphones against anything on the market, NC headphones in particular.
Please read the following accordingly.
I'll start with the good stuff:
I use the AKG at work in an office room shared with 3-4 other persons. This means lots of talking, lots of keyboard typing, computer fan noises as well as air conditioning noises. My colleagues, well... they talk loudly.
The total amount of noise is not terrible. I can easily imagine a workplace setting much much worse, but it is just enough to warrant some peace of mind when struggling with buggy excel macros :)
First of all I really want to comment on something that these headphones truly shine at: low volume listening.
With NC turned on even the lowest volume will produce just the same audio quality as a higher volume would.
This is amazing for me as I prefer lower volume. With many headphones I tend to turn up the volume to be able to hear all frequencies properly. These cans are very nice to my ears even at the lowest possible volume. A huge plus!
The AKG are excellent at removing the kind of sounds I mentioned. The only thing I hear from external sources are some sounds in the [edit: mid to higher mid] treble range, especially parts of human voices.
With that said, talk from my colleagues does not bother me. The lows and highs are clearly cut off. What remains of my colleagues chatter is a highly muffled sound of speech. When I'm not listening to anything I hear that they are talking but I would have to focus very much to actually understaind anything they say.
When I actually listen to something with the headphones I am essentially audibly shielded from any and all "office sounds".
Comfort is really nice. They are heavy but still not too much to be awkward. But tilt your head forward fast enough and they will start to fall off.
Now for the not good, but not bad stuff:
With NC turned on there is audible noise. Like white or pink noise at very low volume.
I find this noise to be quite pleasant actually. The noise itself does not bother me at all and sometimes when the music stops playing I find myself sitting there listening to noise for a good 10-15 minutes before I change track, without any sort of discomfort.
This hiss/noise is not audibly present to me while listening to something, only when nothing is playing.
I just wanted to mentioned that since you seem to be interested in the audio quality of these headphones.
The 40 hour battery life is in my experience not true, as this would equal to about one work week for me. I haven't measured it but I can definitely say that it's not 40 hours. A wild guess would be 20-25 hours in my experience.
However, the recharge is super quick! From the USB port of my work computer as well as my home computer they charge fully in less than 1,5 hours. And by "fully" I mean that the red "charging" LED lamp turns off.
Then they are good to go for another ~22hours. I find this relatively short charge time wonderful. If they run out just before lunch I can plug them in during lunch and when I come back I know I have PLENTY of battery life.
With NC turned off they sound almost the same but slightly attenuated in the lower frequenzies.
A big problem I find is that with NC turned off you hear muscle movement. Say you are chewing a bubble gum with NC turned on and then turn it off - instantly you will hear your jaw muscles rumbling and mumbling. It's actually so much that I never use these cans with NC turned off.
Now the bad stuff:
Comparing the NC on and off, in my office, I find that the NC mostly cuts off the higher end of lower frequencies. It seems that the closed back and decent seal towards the ear does most of the work at cancelling outside noise.
They do a good job at cancelling outside noise even with NC turned off. The problem is what I wrote above - you hear your own muscle movement.
Now if you sit still and rarely move this may not be a problem at all. But I keep hearing this roaring mumble every time I tilt my head or swallow or sigh or take a deep breath.
With NC turned on they remove all of these "muscle sounds" and the only thing you hear is the hiss/noise mentioned above.
Is this a bad thing? To me it is, as it gives you a feeling that you are paying a high premium to cancel noise that is generated inside the headphones.
If you only intend to use them with NC turned on, like I do, then this is of course irrelevant.
As for sound quality it gets interesting. I'm really diversed in my opinion when I think of these cans in terms of quality audio.
Sometimes I find them quite balanced and accurate, sometimes I find them bloated in bass response and highly attenuated in mids and highs.
I have tried them from several different outputs. Xperia Sola cell phone, computer>Fubar IV DAC>O2 amp, computer headphone out, nexus 7 tablet...
I just can't pinpoint what is wrong. Actually, I can't even say if there is something wrong.
It's just that sometimes I find the cans to sound very dark and "sloppily bassy", and just as I am about to mentally describe them as such I put on a different track and I change my mind almost instantly.
It's very weird for me. I can't dwelve any further into this since I haven't got the skills or time to do so.
Soundstage is not much to talk of even though there is some of it, being a closed can that is.
Bass is... all over the place. It's not terribly bassy, there is very little definition in the base frequencies but instead the bass is wide and big.
Like I wrote initially: If you are after high fidelity audio you are guaranteed to find something in this price range that is miles better.
Whether the K495 NC are good for you as a noise cancelling headphone... well I can't say. If this description helped you in any way then I'm happy!
Edited by bestuser - 5/10/13 at 1:23pm