From the G-17 FAQs:
You can easily adjust the equalization for the desired amount of bass and treble when playing music from your Airplay-equipped iDevice by adjusting the equalization directly on that iDevice. To do this, go into “Settings” on your iDevice, then choose “Music”, then choose “EQ”. The G-17 was designed to perform at its best when the EQ preset is set to “Off”, which is the first (and the default) option. However, if you would like to customize the sound, there are also a number of EQ presets with different levels of preprogrammed bass and treble you can choose based on the type of music you are listening to. If you desire, you can sample each one of these settings until you find one that sounds best for you.