There a really many standards with USB. A USB 3 (type A) is required to support 900mA but most modern ports supply BC whch gives 1.5A. A USB C will give 3A at least (when using a C to C connection), but most ports supply 60W, which is obtained by negotiating higher voltages. It gets trickier with USB-A to C cables, here the upstream A port is the limiting factor. On the other side a USB-2 port can strictly _limit_ current to 500mA, such a port would probably require the use of the external PSU with the babyface at full power. These ports are not common any more, especially on modern audio PCs. RME specifies the babyface with 600mA, which is slightly above the USB-2 standard but will work in most situations without the PSU. This is a conservative design approach which will get you running with 99% of audio PCs, but by limiting the power consumption (and thus the headphone amp power). I personally would prefer a higher power consumption of the device with more power available for the headphone amp, and be required to use at least a USB port that can supply 7.5W or 15W. I'd guess eventually RME will adapt to the changes in host hardware to stay competitive. Their conservative approach has its positive sides too (stability and compatibility)!