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My favorite amp under $500 would probably be the new Dynalo going for $300. It gives you 1W of power and it's about the most neutral and clean circuit out there. Even better, the design has been kicked around the DIY community for years. If there were any problems, they've been weeded out. It might not be the newest circuit, but that doesn't matter. Gilmore is a great designer and lots of amp nerds have been through it.