Voltage and current rating for the micro iUSB 3.0 are fairly uncritical. The key is that the external power supply must provide enough power (voltage * amperage) to power any devices drawing power from the micro iUSB3.0 and leave some reserve for losses and internal power use (say 1.3 factor for reserves). Voltages from 8v to 16v nominal are fine, an input voltage of 18v should not be exceeded.
If we want to power two USB 3.0 devices we need 5v/0.9A X 2 or 9 Watt. Add some reserve and a 12 Watt power supply, which could be for example 12v/1A or 9v/1.5A. This is the minimum needed for USB 3.0 compliance. Internally the micro iUSB 3.0 can handle up to 6 ampere total current on the 5v bus, so for an extreme case, a 40 Watt power supply (12v/3.33A or 9v/4.44A) could be used without issue. However, such high powers are unlikely to be needed in Hi-Fi systems and there is little to no benefit spending a lot extra for higher power ratings.
