Register/certified letter first, but be clear in the letter about the next steps, which is small claims court. It It might be a pain for you, but for him it will be a major issue with having two jobs. You can always cancel the small claims court appearance (he cannot) if he gets the amp back to you before the court date, which he will do with the thought of spending most of a business day in court, in which he will lose anyway. And make it clear the claim is for $1,500.00 plus your expenses, not just the repair cost. Most small claims go up to $5,000.00 these days, court costs, your travel and time, etc., your state may be different of course.
I cannot image the police doing anything in this situation. He really hasn't stolen anything - yet, you gave him the amp and you don't have a specific return date. He could keep it for 5 years, and the police still wouldn't considered it stolen.