Amplify Raspberry Pi Zero W Audio with 60s Tube amplifier circuit
Each channel has its own PWM signal, which is pulse-width modulated in the low MHz range. You can either run this digital signal into a Class-D amplifier, then filter the amplified result with a bulky LC-Filter, or run the digital signal into a tiny LC filter, then amplify it with a standard Class-A or Class-AB amplifier. The latter matches your use case.

If you only ever need a mono output, I recommend only using a single PWM, and do the downmixing through software.