Audio Balance issue

Hi,

everytime i turn on my iMac my audio balance shifts slightly to left...

this happens only with external speakers connected to the output jack. When disconnected, the output in pref panel changes from headphones to integrated audio and the balance go to center, but when I reconnect the speakers, the balance moves again to the left.

i got this problem since Lion (10.7.3 maybe, i don't remember), so i updated to ML but the problem persists.

I already tried SMC reset, PRAM reset, "killall coreaudiod" from terminal, deleting audio preferences but nothing works.

It's very annoying!