[squarefoot]

> I could almost swear that you can even create a class-D amplifier with the ATTiny85, with a clock rate of ~15kHz.

Pretty sure you can do that. A Class D amplifier is to a class A/B traditional one what a switching power supply is to a linear one, so I would expect to be able to make a class D amplifier using for example a PWM driven H bridge usually employed to drive electric motors, or a chip used to make switching power supplies. If they can work at high frequencies (say 50KHz or more) then they should work; all it's needed is modulating their input or feedback pins the right way. Probably not HiFi for the sensitive ear, still good enough.