Thats nice to have an optical out from the head. The only reason I asked was to see if you were using speaker lines out to the amp. Since they carry power and you have to run them into the low level input sometimes the juice from the head can over-power the amp inputs. But since this is not the case no need to worry about that, optical out is definitely the way to go.
Try turning the amp down a bit, like you said and let me know if that does the trick. If it doesn't I may be out of ideas. Good luck.