The Icon USB DAC has a maximum output of 0.5V, while the uDAC is designed to have an output of 2.0V. Why? Icon is an integrated amp, and at peak 0.5V USB output, the power from the Icon reached the maximum. That's why.
However, uDAC is just a line level output, and it's designed to behave like a CD or DVD Player to have a 2V output.
uDAC + Icon Amp will have more volume and dynamic than a Icon (using build in USB).