The ASUS Xonar Essence STU comes in a huge box. The unit is very well protected inside the case.
The bundle is quite extensive. You get the charger, USB cable, and stand.
The PSU is quite nice. It never runs hot and is small.
The STU's case lacks some of the finesse the Xonar One's has. The exterior of the STU is a plastic affair. All the dials and buttons are of a really good quality, and there is no static on any of the volume controls.
These relatively large feet make sure the STU stays in place. They are well-designed, decoupling the device from the surface and absorbing a good amount of vibration.
One interesting thing about the STU is its dedicated headphone out. It is run by the same chip (TI TPA6120A2) as in a STX. The chip has excellent specifications and powers high-impedance headphones to perfection. Its only issue is the recommended resistor value—its default output impedance of 10 Ohms adds some distortion to low-impedance headphones. The output impedance should, as a thumb of rule, be at least a factor of 6 lower than the impedance of the headphone. TI's recommendations are based on stability concerns, but quite a few companies and skilled DIYers have managed to overcome the problem, producing TPA6020A2-based designs with output impendances in the 1 Ohm region.
You can feed the STU through both S/P-DIFF and USB. A button at the front allows you to pick one or the other.
Inputs are bundled up on the backside.
Much like the Essence One, you get both a headphone out and real line out. The real line out has a separate volume control, which is neat with a set of studio monitors at home.