I agree with @agedhorse's call for some compassion; I ran my own little business for about 40 years and I'll have to admit that more than once I made business decisions or said things in the heat of the moment that I regretted later. And I empathize with how it can get to be in a one-man show--sometimes "there's just not enough of me to go around", but my empathy only goes so far. I'd bet that it became obvious to the builder he wasn't going to build that amp a year or more ago, but he lacks the liquidity to refund the money--a status also not completely unknown to me. For @Low Class, I'd offer this: I'm guessing now that you're regretting having made a 100% advance payment. I don't blame you too much for having done so, since the builder enjoyed a good reputation here on this board, and this a trusted community, but personally I would never make a full payment in advance for something that hasn't been built yet. There is a local builder who has what I feel is an excellent protocol: you put your name on his list at no cost; when he's about 2 weeks away from starting your project, he contacts you and collects 50%, with the balance due on completion. That sounds like a pretty good way to do it in my book.
Anyway, I'm really sorry for the loss, but I don't hold much hope for your being made whole...it does indeed sound like he's planning to exploit the fact that it isn't a large enough sum to justify getting lawyers and courts involved. And I guess, if he's 'retiring', he isn't that concerned with the hits to his reputation; that's too bad--it seems he had a pretty good rep prior to this.
And once again, props to @Low Class for having shown what I feel was exceptional patience before giving up on the guy--it's too bad your patience wasn't appropriately rewarded.