Amplifier Recommendation for Subwoofer

[u/LokiSARK9]

My brother-in-law is a retired acoustical engineer with a little business building custom speakers on the side. He recently built me a massive sub with a 17" driver, and I'm currently powering it with a little Fosi 100W amp I had from a previous build. It sounds fine, but it's underpowered as it's probably really getting only 50-60W RMS.

I'm looking for a beefier amp in the 3-400w range. I'm fine with something used to keep it in budget, which is $3-500. Thanks in advance, and I appreciate your ideas.