Some hiss from the FRFR speaker cabinets is normal and may be audible when no audio signal is present or at low volumes. This is a natural hiss and should not be affected by the level of the input knob or contour switch, though be sure to throw the ground switch on or off to make sure no ground hum is present. 

For the best results, check the following:

1) Make sure all cables in your signal path are good quality and confirmed working.

2) Connect the FRFR and connected devices (i.e. HeadRush Pedalboard) to a power conditioner.

3) Turn up the master volume level on your Pedalboard or other connected devices to ensure the input signal level is strong.