If I plug into the Aux input can I get full output power?

Absolutely. Plugging into the Input delivers power to the output however do note that it bypasses the preamp and functions as a 50/50 mix with the guitar channel by default. This means that any volume adjustments into the input must be controlled externally. The 1/8 inch Aux in is a 20K impedance input so it will work well with most keyboard or MP3 player type inputs but would not be a good choice for instruments.