What To Do?

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.

@nicholsr Just another idea:  First off, nobody is that busy that they can't spare a minute or two to float you a quick call and/or send you a quick text message providing some sort of information on the status of your repair.

Secondly, the idea of sending written communication via registered mail or the like is a very good idea.  If push comes to shove, this documentation will seal the case in small claims court.  However, if it is accurate that mail is not received at this person's shop, which I doubt, and you do not have his home address, then you could try this:  You could contact the police in the jurisdiction of this guy's repair shop, explain your dilemma and ask if they would be so kind as to give you his home address.  If, for some reason (e.g.  confidentiality/privacy; etc.), the police wouldn't or couldn't share his home address with you, you could ask them or the police in the appropriate jurisdiction to do something called a "wellness check" to make sure this guy is OK.  In the process, they would explain why they were there and why they were checking up on him.  If need be (e.g.  small claims litigation), this police report would be good documentation, as well.

P.S.  Keep track of your expenses in trying to smoke this guy out.  This would also be claimable in small claims.  Good Luck!

small claims is bogus where your lawyer may have unreasonable attorney fees towards the price of the equipment that is extremely questionable and judge would not issue you full refund of attorney fees. it's a game of a chance where you likely will just lose and the odds are not in your favor by very far even if you are 100% right.

spending an hour to drive there might be essential, because you still want to see if the place is even being attended. maybe it's already abandoned and there's an eviction notice or many-many factors merge... In case of eviction notice, you can buy the entire equipment for nearly-NOTHING and make profits

I'd send him a letter with receipt conformation stating date and time when you will be at his place to take your unit back home.

That’s exactly the reason I gave up my vintage gear except speakers. No one left who repairs them as a main business. In 2021 I waited 4 months for a minor repair with communication only when I forced it. No more. Good luck.