i see 3 options.
1: listen to steve and call the police, shouldn't take them long to solve the problem.
2: find out his schedule, make sure you are by his car when he tries to go to it, and be leaning against it with a baseball bat resting on your shoulder. This one is all about attitude, you have to be VERY confident and convincing that he will decide it is better to give the amp back rather than call your possible bluff. risk is he will come out with a weapon too (although i doubt it).
3: if you know how to do it correctly then just pick his locks on his door (or a slim jim) and just steal it back, what is he going to do? risks are numerous on this one (law breaking, injury, jail, retaliation, etc).