According to Thomas G. Marshall . com:

Seems like either the controller or the controlled is gonna buzz when at 1/2
power.

Switching bulb styles and/or manufacturers, perhaps trying out a 100W rough
service bulb is a good first & cheap step. Get away from the ultra cheapies.

If the dimmer is an ultra cheapy, switching to a $20-$30 (I'm fond of Leviton and
Eagle sliders) unit will sometimes resolve a problem that switching bulbs won't.

Is there no way for these things to have a variable transformer that reduces
V amplitude while maintaining the waveform as a (roughly) 60 Hz sine? Or
would that buzz as well?

You don't want to know how big, heavy or expensive a 600W variac is.

They don't fit in a receptacle box.

And they make electrical noise as they get older.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.