How about:
Multiphase Cuk converter.
A real easy design is to take a cheap, off-the-shelf class D amp chip,
and put an oscillator in front of it to drive your output stage with a
nice sine wave. Put a feedback loop in to push the "volume control".
Make a super clean, low ripple linear fed by what is technically a
switcher inside the amp chip. Even if you only run it at like 17kHz, the
chip is at like 400kHz sample rate inside. This 'noise' can be detected
in your output if not managed properly.