[nongrata]

I always was and still are a huge fan of the Sennheiser HD-540 series. 
During the years I heard many headphones but they remained my favorite.
Even the nowadays toppers like Sennheiser HD-600 did not sound as good according to me.
When I read in the head-fi forum that someone liked the AKG-K340 more and I read all the reviews thatl were so positive I got curious.
I found one on Ebay and bought it but because I never liked the (new) AKG-242II I bought a few years ago I did not expect them to be better then the HD-540 series.
To my astonishment they sounded really great.
After an hour of listening and comparing I had to admit, they are indeed better then the HD-540.
Now a month later they are still the headphones I use.
The only disadvantage is they are less comfortable to wear then the HD-540 which is very comfortable,
The sound of the AKG is however fantastic, so realistic, you hear all details and the bass, mid and highs are extremely realistic.
The soundscape is beautiful, there is a remarkable space between the instruments and the AKG can do something no headphone I know can.
Even the Dolby sound headphones or software there is that should give a 3D like sound can not generate this depth.
I never liked the artificial 3D as it is more like a reverb you hear and the music sounds strange and blurred.
But the AKG K340 can create an image that is 3D like without a trick and sound as pure as it probably was in reality.
You need a quality amp that can drive them as they are very power hungry.
I use amps from the seventies and they have juice enough for them.
If you have not a powerful quality headphone amp you will have a Ferrari on a field track. 
I never heard the Sennheiser HD-800 so they might be better (cost a fortune) but of all the cans I heard the AKG-K340 is te best.
Mine are original, not modified, I tried them with the removed grid (installed a selfmade grid and without) but the highs were exaggerated and the mids were not clean anymore.
So original they are in my opinion already perfect.
Why are headphones from decades ago so good ?
Try them, you can buy them second hand for not to high investments but you will get an amazing quality in return that I personally doubt you will not find
nowadays in a store below 500 euro.  
 

 

[vid]

The 340 are good in the treble, ie. the domain of the electret, but the rest of the spectrum is not up to it given a wonky response and outright poor decay, which you'll find in any measurement of the 340 ever done. Interesting soundstage, but should be priced no higher than $50 is my take on them.

 

[nongrata]

Of course all is a matter of taste so I respect your view.
But in my opinion when it comes to measurement, i think that when the manufacturers started to try to get the best results in measurement at  end of the seventies the musical pleasure amplifiers could generate disappeared.
A modern amp has probably less distortion then a seventies transistor amp and is a lot better then the tube amps of the sixties. 
Still they old amps sound much more musical and engaging compared to new ones.
The old ones sound like you are there, the new ones are cold and sound distant.
The K340 does has a great bass, yet you only hear a bass when there is a bass, but this is how i personally like it.
I would say the K340 (or HD-540) is more suitable for classical music or music that needs a headphone more neutral but they even sound great with modern music like club/dance. They go really low, without sounding muddy.
If you buy new, a $50 headphone will sound like crap compared the K340.
If you can buy them for $50 second hand, go for it, you will not be disappointed (even if you pay $100).
Please note, you really need a good amp, modern ones with an IC to power the headphone will not let them shine at all.
Buy an amp made before 1975 or so that will tap the headphone output from the power transistors and you will hear music as if you are in the room it was recorded. 
Like I said before, the 3D image they can create is unbelievable and no other headphone I (personally) heard before can create such amazing sound field. 

 

[Claritas]

Which version is it? What amp are you using?

I have the green driver version and find it needs to tubes to sound right, to integrate the ranges.

I agree about the comfort. Pad swapping might help or not. K271 velours have smaller openings than stock pleathers but don't get warm. But I prefer their sound; I'm thinking they position the drivers more directly in front of the ears, making it somewhat more aggressive.

I'm not going to argue about the price. Let's just say I disagree.

 

[vid]

Depends on personal taste, of course, but from a hifi standpoint, the 340 are overpriced compared to what else you can buy. Though I don't think the 340 were ever meant to be a hifi headphone anyway, AKG's dynamic phones from that era were more for that purpose, giving a smoother response with better detail in all but the treble, costing only a third of the 340 today. Some vintage planars you can have for as little as a sixth of the price, with less distortion and quicker decay whichever way you look at it, if usually lacking in soundstage. A modded sennheiser unipolar is less than half the price, with by far more potential and in my opinion the more immersive soundstage. But even in the current price range of the 340, you have the fully electrostatic sr-5, which though doesn't have the soundstage, comes with a more controlled response by a fair margin.

 

[hatrack71]

These are ones I gotta hear. I have the K270s and they are supposed to be a taste of the K340.

 

[Audiogalore]

  I always was and still are a huge fan of the Sennheiser HD-540 series. 
During the years I heard many headphones but they remained my favorite.
Even the nowadays toppers like Sennheiser HD-600 did not sound as good according to me.
When I read in the head-fi forum that someone liked the AKG-K340 more and I read all the reviews thatl were so positive I got curious.
I found one on Ebay and bought it but because I never liked the (new) AKG-242II I bought a few years ago I did not expect them to be better then the HD-540 series.
To my astonishment they sounded really great.
After an hour of listening and comparing I had to admit, they are indeed better then the HD-540.
Now a month later they are still the headphones I use.
The only disadvantage is they are less comfortable to wear then the HD-540 which is very comfortable,
The sound of the AKG is however fantastic, so realistic, you hear all details and the bass, mid and highs are extremely realistic.
The soundscape is beautiful, there is a remarkable space between the instruments and the AKG can do something no headphone I know can.
Even the Dolby sound headphones or software there is that should give a 3D like sound can not generate this depth.
I never liked the artificial 3D as it is more like a reverb you hear and the music sounds strange and blurred.
But the AKG K340 can create an image that is 3D like without a trick and sound as pure as it probably was in reality.
You need a quality amp that can drive them as they are very power hungry.
I use amps from the seventies and they have juice enough for them.
If you have not a powerful quality headphone amp you will have a Ferrari on a field track. 
I never heard the Sennheiser HD-800 so they might be better (cost a fortune) but of all the cans I heard the AKG-K340 is te best.
Mine are original, not modified, I tried them with the removed grid (installed a selfmade grid and without) but the highs were exaggerated and the mids were not clean anymore.
So original they are in my opinion already perfect.
Why are headphones from decades ago so good ?
Try them, you can buy them second hand for not to high investments but you will get an amazing quality in return that I personally doubt you will not find
nowadays in a store below 500 euro.  
 

I totally agree with this assessment the AKG K340 is one of the finest and most natural sounding phone ever produced. However does need the right amp to drive them properly to bring out the details as mentioned. If you use a improper amp it will sound like crap, garbage in garbage out
Amp recommended would be the SPL Phonitor.