*beni23
Wala ba sila mga touch-up lang? Then again, that's hard to do as the scratch's pretty deep.

Personally, I'll just leave it as it is. It might piss you off at times but I believe there isn't any cheap "fix" to restore it back to its original state. I'll try to pursue John's advice in the vent panel and hope it's detached from the body (doubtful here too)

*JohnM
I would suggest EQing the treble and mids kung nasasapawan na ang highs. Although my usual suggestion would be to lower the EQ (maxing it out isn't really all that great) and try to make it up by more power (increase the volume knob). Ex. If your usual setting's 10/10 bass, 0/10 mids, 0/10 treble at volume 10. Try to aim for 5/10 bass, 2/10 mids, 4/10 treble and volume 20.