I wouldn't take the comments by GOHOINC the wrong way. He's not slighting you, diminishing the situation, or trying to trivialize the ordeal. He's just trying to give an honest assessment and objective opinion. People here are trying to help. From a purely legal and business advice perspective, I agree. As terrible as the situation is, and as big of a PoS that the buyer and all like him are, there's not much legal recourse here you can pursue. I doubt any reputable attorney would take the case after an initial consultation.
What would the lawsuit be based on? What exactly are you trying to sue based upon? Yes, he's a douche hole that did you wrong, and there's no one with a half a brain in the guitar community that would side with him...but the guitar community and the legal community are two separate things. The Cliff's Note version (to a lawyer) is that you sold an amp to the guy online. He went through the proper channels and claimed there was an issue. Those proper channels awarded in his favor, and he's returning the amp. Assuming you get the amp back (I know it's in transit), what's the claim here? They've already noted he provided sufficient evidence for a decision.
Even if he swapped out components and such, do you have any definitive proof that the amp wasn't like that before you shipped it? Gut shots, videos, anything to validate the claim? Even if so on that end, what sort of restitution would you be seeking? Any monetary award (shipping fees perhaps?) - which likely wouldn't go in your favor - would probably amount to only 1-2 hours of a lawyer's time. You'd go down the rabbit hole and light a match on stacks of cash. Would he suffer any because of it? No, but then he'd probably threaten slander/libel and cite threads such as this one right here.
No one here is disputing you. You are objectively, 100% correct in the eyes of everyone here. It's messed up, everyone agrees with you without question, and it probably inflames and infuriates every single person who reads the story. But, you have to remember that our barometer isn't the same that the legal community would use.