All audio electronics including audio interfaces, studio monitors, mixers, PA speakers, even a stereo all have a hissing sound if you turn up the output all the way. You are hearing the noise floor of the device. The best method to eliminate or greatly reduce this issue is to set your speakers to Unity which on the Eris speakers is at 12 o'clock. There is a notch in the rotation of the knob indicating the regular position where it should be.
You can then use the gain knob to adjust up or down the loudness of your monitors. Keep in mind that turning up your audio interface inputs and/or outputs all the way will also reveal it's noise floor. By adjusting the output of both your audio interface and fine tuning of the setting for you monitors should get you to a setting you're content with.
An internet search turned up this simple to follow guide on how to setup your studio monitors. He goes over the basics of setting up studio monitors and setting levels. Check it out.