I bought a Boss car audio amp (BA-4800) for a staggering $2. There was noticeable physical damage, but it’s worth a gamble for a quick, cheap fix.
The damage looked localized to two RCA connectors. The remaining pins from the connector looked undamaged. Without changing anything it was powered with a PC PSU. Volume was good from both the remaining RCA connectors.
Desoldered the pins and replaced the RCA connectors with an additional through-hole quad RCA connector.
Connected power again: Now all 4 channels were working well. For what started as $2 for 2 working channels is now $6 for 4 working channels. Thats $1.50 per channel.