The Mosfet output transistors themselves as well as the output filter, which both have not changed much since it’s inception.This statement is 100% and certifiably false.
The GaN output transistors are maybe going to change that with optional far higher optional switching speed available, so then output filter can also be far higher up, then there’s also no phase shift down into the audio band. But this also needs small heat sinks to be used, as you can see below.
George, I suggest you read the white papers of Bruno Putzeys. Advisory: its is helpful to know a bit of calculus to follow along.
Your statement about the filter is incorrect if the amplifier is self-oscillating; even if the filter frequency is fairly low (80KHz for example) the feedback used in almost any self-oscillating class D amp allows it to correct the phase shift from the filter from affecting the audio passband.
You don't use GaN devices to increase the switching speed- it appears that Technics did it purely for bragging rights, not sonic merit. You use GaN Devices for their properties such as fast switching speeds, gate input capacitance, output capacitance and possibly lower deadtime. 600KHz is a very practical upper limit for switching with them, not because they can't go faster but because creating a low noise layout gets exponentially harder as frequency is increased. But you should be advised that MOSFET devices have been fast enough for a while that Technics could be easily using them at the same switching speed; Bruno is using MOSFETs rather than GaN devices in his Purefi circuits- think about that for a moment will you? Bruno is arguably the acknowledged master in the world of class D and he's not using GaNFETs. This is not to say that he doesn't acknowledge their benefits. Quite simply it just isn't as you've portrayed it above.