Big problem with USB power sources is even at 400mA some will drop voltage. I have seen ones rated for 5V 1A which go down to 3.75V at 700mA.
I have a few well-built ones which will provide 5.01V or better at 1500mA draw. I use these for power sources, with appropriate barrel connectors, for various things around: Sega Genesis HD, CF7+ and nanoPEB (TI,) Atari 130XE, etc.