[Fandangoya]

I've had a pair of HD555's for about two years now, and I'm unsure if they have stopped working to their full potential.   It sounds to me that the sound stage has been shifted slightly to the left, but it very hard for me to tell.  I listen to a lot of live concerts and sometimes the mix can do that as well.
 
Anyway, is there anyway I can test the soundstage and test if I've lost some output in the right speaker or something?

 

[Soaa-]

Try wearing them backwards to see if your perceived unbalance shifts to the other side.

 

[NapalmK]

What I do is go into the sound settings and change the balance until it the volume of both sides is equal. It should give you an idea of how bad the imbalance is. Of course you could also wear them backwards as Soaa suggested.