Hello there!
 
I recently purchased a pair of n40s and am extremely happy with my purchase aside from one irritation. There appears to be a frequency, or one set of frequencies, which cause a highly irritating crackling noise to interrupt the track that I am listening to. The instrumental section which begins at 5.35 in Led Zeppelin's Stairway to Heaven is a very good example of this effect - the track plays (almost completely) flawlessly up to this point, but when it gets to 5.35 the sound begins going absolutely crazy with crackling, distorting the track and making it almost unlistenable. The same effect can be heard in certain places on the same album in the track 'When the Levee Breaks', among a variety of other examples including some of Nick Drake's instrumentals. I am driving the earphones mainly through audioquest's dragonfly red, though I have isolated the problem away from this by trying to play these songs directly through the laptop's soundcard, on my phone and even on an old Sony MP3 player. I have also isolated it away from being an audio source problem - on my laptop I am listening to the tracks (mostly) in FLAC at 24 bit, 44.1khz, but have listened to the same song on spotify and even on youtube and all produce this same effect. Changing the volume makes no difference to the severity or existence of the effect. I have even ensured that the problem is not a physical one related to my ears by asking others to listen! Thus I must imagine the problem lies with the earphones themselves, either the IEM driver or the cable. I would like a little advice as to what I should do. 
 
Thank you, and apologies for my English - not my first language :S

First, check that the earphones are clean. Perhaps some earwax is causing unwanted distortion by resting on the driver or creating unwanted oscillations of the driver by plugging something up.
 
Second, there are different types of filters you can put on the eartips to tailor the sound. I recommend swapping for the bass boost option and see if those filters remove the distortion.
 
My last thought is the impedance of the in-line mic is causing some problems. It would be awesome to swap out the cable for one that does not have the in-line mic and see if that clears things up. If the impedance is not linear or has some giant peaks in it the source will not have full control of the signal and distortion may develop. This is typically heard in the bass, but it could affect the treble too.
 
You'll want to make sure the music you listen to is not low quality MP3 or poorly recorded because most good IEM's tend to reveal the flaws in recordings. Take a listen to the song in the link below and tell us if you hear distortion in it.
 
https://www.youtube.com/watch?v=_rRXFyWFveQ
 
It will help eliminate a few things.

Hello! Firstly, thank you very much for your response - it is appreciated. 
 
Unfortunately, I had already tried switching the cables - to no avail. Though these earphones are only a week or so old and therefore unlikely to be affected by wax damage, I did try cleaning them just now and this unfortunately has also failed to fix the problem.
 
Re the source material, as stated above my files are high resolution FLAC, and to make sure it is not a source problem I've tried streaming from spotify and from youtube also. The tracks in my FLAC library which present issues similarly cause problems when listening to these other formats and so it is very unlikely to be a source issue.
 
I'm starting to get a little downhearted. Since these have been bought in China from a third party seller, they are not covered by an AKG warranty and so cannot be sent for repair
 
Thanks again in advance for any help or other suggestions anybody might have.
 
Edit: It would appear that the bass boost exacerbates the problem and the high boost perhaps slightly alleviates the problem, but not to any great extent and I actively dislike the sound that both filters produce.