with a popular choice being headphone amplifiers that use a tube as a gain stage followed by an op-amp as a buffer with a low impedance output. This forms the basis of [Ratti3]’s amplifier, but ...

[Simon] wrote in to tell us about a headphone tube amp that he just built ... The case for the amplifier is laser cut plywood. The top piece is kerfed so that it can bend around the radii ...

There are a couple of reasons I suspect it's taken this long to see a vacuum tube in a pair of headphones. Tubes are hungry ...