I think there are way too much credits given to "USB DAC".
Other than the low end USB DACs which actually use USB DAC chips, many "USB DACs" are just "regular" DACs with USB interface built-in.
There are certainly theoretical advantages in a tightly coupled USB interface, versus an external USB/SPDIF box. But a DAC is still a DAC. The DAC chip and the analog output stage are still crucial factors in producing the sound.
Other than the low end USB DACs which actually use USB DAC chips, many "USB DACs" are just "regular" DACs with USB interface built-in.
There are certainly theoretical advantages in a tightly coupled USB interface, versus an external USB/SPDIF box. But a DAC is still a DAC. The DAC chip and the analog output stage are still crucial factors in producing the sound.