This is the polarity on the plug if I do understand correctly it's not the right polarity?
If you compare that to the boombox, which has the same symbol next to the jack, the polarity is reversed. DO NOT plug that in, you'll probably blow the boombox or the power supply, or both.
You can still use it if you cut the wire near the end, and reverse the wires. And test before you plug it in. On your boombox, the center pin is (-). On your power supply, the center pin is (+). You need it to match the boombox.
You can also open up the boombox and reverse the connections to that jack, which is the more elegant solution and does not require cutting the supply cable of the power supply, but then the molded polarity indicators at the jack would be incorrect and a new or future use, or even yourself if you forget, might plug in an incompatible polarity supply and fry something.