At what level of Streamer and Dac does a DDC add no sonic benefit?

I use Innuos ZENith mk3 USB into U18 DDC and I2s to Audio mirror Tubadour V dac. The difference vs USB direct to dac is not subtle. Unbelievable indeed.USB is extremely muffled in comparison. The veil being lifted with I2s completely applies in this case.

Results variable depending on quality of interface/connection in streamer, dac and DDC.  I too have found I2S superior to usb input on Laiv Harmony and Musetec 006, this with Denafrips Gaia DDC. Gaia has very nice OXCO clock with quality power supply for clocks.

I have a Hermes sitting in front of my Pontus II 12th and it made a difference.  I don’t know at what point it stops making a difference but it made one at the level of my components.  The clock in the DDC is better than the clock in the DAC so that certainly plays a part as well as using the I2S connection.

I have not tried a DDC. But, I am a big proponent of having the very best components you can afford as opposed to adding things like external power supplies and I think DDC would fall into this category. When I have tried this, it has not been as cost effective as getting a better component.