Class d would be the smart option if you were building a battery powered headphone amp.

The audio decoder chip in the old first generation airpods had a class d (trade name class h) amp good for a couple of watts into four ohms in it. Iirc the ones used after that aren’t much different.

There’s so much tomfoolery going on with any signal coming from a smartphone you’re probably barking up the wrong tree worrying about the type of amplifier in your headphones. Between the software decoding and applying all sorts of effects to the source signal before it’s possibly transcoded to something the bluetooth chip can easily handle and manage buffers of then picked up by the processor in the headphones, possibly further effects applied then finally passed to the audio codec chip that can apply a final set of nyquist stuff to it before driving it into the actual speaker.

The sound quality out of switching amps driven from batteries though is pretty phenomenal in my experience.