Fosi Audio TP-02 Subwoofer Amplifier Home Audio, Mini Bass Amp Mono Subwoofer Amp, with TDA7498E Chip 220W Volume and Frequency Control, for Powered Passive Subwoofers Bass Shakers

A Crown XLS Drivecore 2 might do the trick. Built in parametric EQ too so you could dial it in. https://www.crownaudio.com/en/products/xls-1002 Not expensive per se but $400 or so off Amazon, so not free. Or maybe https://www.amazon.com/Subwoofer-Amplifier-Integrated-Fosi-Audio/dp/B07L6FN97Z - lesser in every way but maybe enough.

I used one of the Fosi sub amps on a Paradigm sub-woofer where the internal amp died.Worked perfectly and less than $70. Ran the amp output directly to the internal speaker. Fosi Sub Amp