What To Do?

Are you in the US? If so:

If you're unwilling to take a drive to see him, send him a USPS certified letter, return receipt requested. Send an identical copy of the letter via First Class mail, noting on each that it is being delivered both ways. (This way, if he doesn't accept the certified notice and retrieve it, he's assumed to have been noticed because of the First Class copy.)

In the letter, summarize your experience in detail.

Then issue a deadline, e.g. "I want to pick up this amplifier from you with 7 days of your receipt of this letter. Should that not be possible, I will take further action."

He'll probably cooperate. If not, you have choices. How much is the amp worth? Small claims court might be a better option that the cops.

If you want to temper with vintage audio, the only way you have now is to become tech on your own...

I should clarify even further.  To my knowledge he only takes appointments to meet him at his shop. I'm sure he's there at other times, but i know he has a full-time day job. So, my chances of seeing him at the shop are very low.  And, of course, I can't make an appointment because he won't return my correspondence.

cleeds' advice is very good, I think.  Problem is, I only have the shop address, and I'm sure he receives no mail there (it's way out in the country, in a metal quonset hut). 

So, he only uses the shop to do his tech work (lots and lots of audio equipment and tvs laying around). It's not a registered business (cash only) and has no name.

The amp would probably bring up to $1,500 if sold on a site like this one.

Have you gotten your friend who recommended him involved.

Does he live any closer? Perhaps your friend could reach him, make an appointment about one of his pieces, then he takes your unit, or you go with him, take your piece.

Have a nice visit with your ex-friend while there.

property record for hut = destination of letter @cleeds advised