Well the question is would the caps inside the cube be an issue also. Since the speakers take full advantage from all the power from the port its suspect also.

If you had a turtle beach audio USB dongle it's supported by Mac os and needs no power and you would have a mini jack for headphones or a regular speaker set.

Did you go to apple system profiler and see if the speakers showed up in the USB tree? How about the sound control panel? If it sees it may just be an issue with that board or a chip on it.