Not likely the PSU. My newly built system has a 650w PSU and yet...
I am having similar problems in that some USB 2.0 devices work and some don't. I don't have any USB 3.0 devices to check those yet. I am also having other known issues with the MB.
I think SOME of the problems are solved by updates to the BIOS and drivers...but I am still doing research via ASUS forums and support. However, some of the information is contradictory and some of the steps required for the MB are exasperating; such as clearing the CMOS after doing anything/everything?
I'll report back my results.