According to HRT, the microStreamer has a digitally-controlled analog volume control:
"Dual purpose built, it incorporates both a high–performance headphone amplifier with an analog volume control that allows you to connect the microStreamer between your computer and headphones; and a fixed-level, low impedance 2.25V line output that allows you to connect between your computer and home stereo, other entertainment system or to powered loudspeakers.
Volume control of microStreamer’s OCL (output capacitor-less) headphone amplifier is accomplished by using the digital ‘slide’ control on the host computer’s music playback software to adjust microStreamers’s internal analog volume attenuator. This clever analogue attenuation arrangement allows the full audio quality of the microStreamer to remain pure, clean and full-ranged, avoiding the known sonic degradation of digital attenuators."
Such an analog volume control of the USB DAC/amp is adjusted by a virtual or physical up/down knob of the PC which has an implementation of USB HID (Human Interface Device).
The digital volume control of the USB DAC/amp is adjusted by a virtual up/down knob of a music player running on the PC.
I don’t have a USB DAC/amp which has a digitally-controlled analog volume control, I don’t know if Android device has an implementation of USB HID.
Yes, digitally controlled analog volume. Unfortunately, at least the Android 4.1.2 with the Touchwiz overlay that I am running on the Note II cannot control the volume controls of the Microstreamer. It is ok with my Momentums, but basically unlistenable with my W4Rs. This is why I am excited for the GEEK, which has analog controls on the device with a similar form factor. The only issue with it is that it will be a 32 bit device that might be harder for Android to deal with and it might draw too much power to use without an external hub (defeating the whole point of something so small). The Microstreamer, BTW is excellent with a laptop.