Since it appears you don't have much of a plan in terms of paying down debt, investing, asset allocation, or anything of that nature, I voted for the headphones and amp. Might as well enjoy the money you're blowing. ;)
(This is tongue in cheek for: this thread is ridiculous, don't ask strangers on the Internet where to invest a surplus, ask where to get the knowledge for you to figure out the best plan to invest the surplus.)