How to test for mismatched drivers? Then correct?

[Folex]

Little background, same audio card and same amp used with 15 pairs of headphones. I've had 2 pairs of headphones that would fatigue only on 1 side. I'd switch earcups and the other ear would become fatigued and start to hurt. On both headphones I re-soldered both cups and the plug and had the same results. I then tried replacing the cable and had the same results. Is this due to a bad or mismatched driver ? And if its a mis matched driver is there a way to test it and correct it ?

 

[higbvuyb]

get a tool like sinegen (http://www.tucows.com/preview/502787/SineGen). Sweep up and down the frequency range. It should always sound like the sound is coming from the 'centre'. If it tracks off to one side, you have an issue. 
 
Are the earcups physically uncomfortable? That can cause fatigue.

 

[Folex]

It's the sound being produced.  If you have a pair of low ohm headphones and an amp you can reproduce the feeling I have. Turn the amp up all the way and turn down the volume on your soundcard/dac so thats its a comfortable listening level. After an hour at most your ears should begin to feel really bad. This is the same feeling I get with 2 of the headphones on 1 side just straight out of a soundcard.