No, you are wrong, it doesn’t matter how well each input processes their independent signals they all have to go through the same OP amp output and to your power or pre amp. Therefore you should concentrate on matching your DAC output cables to whatever is receiving them.
This is not entirely correct. USB is a data packet interface. When using this, the DAC has to un-pack the data and then clock the pulse signals at the correct timing and bit depth for the DAC. This is essentially generating i2s data for the dac chip itself.
With S/PDIF, the data pulse signals are already clocked by the transport/source and the DAC just has to separate out the left/right pulses for each channel. This is essentially converting the S/PDIF signal to i2s signals. S/PDIF is a single data stream that combines the i2s information for both left and right channels.
So, if the DAC has a crappy USB receiver module, the sound quality could definitely be better on the S/PDIF or AES/EBU inputs. It all depends on what kind of USB receiver, what kind of clocks it uses and the quality of the power supply for this module.
In some cases (like the LKS MH-DA005 dac) the USB receiver module has been reported to be better than what can be sent over the line using S/PDIF or AES/EBU.
