Hey blu fuz, that's a heckuva score you got there!
Just a thought about that power supply; I would definitely seek out a PSU that is spec'd properly to that boombox, here's why:
If that PSU is DC (direct current); the current rating (in amps) can be equal or even higher than spec'd, because DC powered devices only pull what current they need and nothing more. In AC (alternating current), the current is always constant so that's where you need to watch ratings. Since you are using one that is spec'd lower than what your boombox requires; the box isn't quite getting the juice it needs and you're also straining the power supply. In all probability it will work fine, but it is bordering on unsafe to run it like that too long, especially if you crank it up; because that's where the higher current draw will kick in.
Anybody who knows more about this than I do, please feel free to correct me!