I suspect that the gan fet is a chunk of this progress assuming it is well implemented. I am gonna pick up some class D monoblocks in the near future.....
@jaymark What GaNFETs bring to the table is low noise. You really do want a class D amp to be low noise because that noise can leak out through the air or on the AC power line. It can get into other components and mess with them. So its not enough that the class D amp meet EU Directives for low noise (or in this country FCC 'part 15'); it needs to be a lot lower so things like digital audio, FM radio, a phono section, don't get upset by the operation of the amp in proximity.
A major source of noise in class D is something called 'parasitic inductance' which is might be a lead of a part or a trace on a board that is long enough that its able to resonate and transmit noise like an antenna. GaNFETs typically don't have leads making them inherently lower noise than MOSFETs, which usually do have leads.
GaNFETs also tend to have a very low 'On' resistance, so low that a lot of the time they don't even need a heatsink, because with lower on resistance there is also less energy dissipated in the device. The inherently faster speed of the devices also means less deadtime to accomodate their operation. Deadtime is a period of waiting in the circuit so time is given for one device to turn off before the other is turned on (otherwise the devices can heat up very quickly and fail). Deadtime contributes to distortion- reducing it can make the amp smoother and more detailed.