Sennheiser HD25-1 II different sound from left and right channels?

[milcs]

Hello everyone,
 
I have been lurking around this forum for a while, trying to get some audio wisdom from all of you!
I have had a HD25-1 II since November and recently I have started becoming aware of a potential problem with my headphones. Basically, I have the impression that my left and right channels sound different. I started playing around with balance, turning the headphones around, etc... and I am just becoming more and more aware of the different sound coming from the different earcups.
I had my best impression of different behaviour of the channels, when I listened to a frequency test tone sweep (20Hz-250Hz). I am almost sure that, after a while, my left channel stops giving me increased frequencies (the same when I turn the headphones around, so shouldn't be my ears). 
Did anyone ever experience this? Could anyone advise me on how to test this further and whether to get this fixed? Is this all in my head???
 
Any help is greatly appreciated.
Kindest regards to all,
M.

 

[Xinn3r]

The cable jacks are sometimes loose, try plugging them more tightly...

 

[mazema]

Are stereo sound supposed to be different in the two channels? maybe you can try some mono tracks to confirm.

 

[milcs]

Thank you for both your replies! 
I did try changing the cables, unplugging them and plugging them again... not much has changed.
For the stereo sound, I assume that the frequency sweep I listed to is mono. This was the most obvious proof of different responses to different frequencies I got (as I mentioned, it starts sounding fairly similar at around 20Hz, and has freq increases, the differences start being noticed.

 

[shadow419]

Try swapping the cups and see if the problem switches sides.  This will narrow it down between the driver and the cable.  It could always be source related, but this is easy to check out with another set of headphones.