First check that the cables you are using are not faulty - swap them with ones you know to be good. If they are good and you can still hear noise then ensure they are not running past any sources of RF interference. Used balanced connections wherever possible so as to ensure the noise induced into the signal is kept to a minimum. Cheap cables with a poor screen can often result in an unclean signal.
Check the headphone out of the VoiceMaster Pro to see if the hiss is still present there (make sure all of the processing is switched out when you test). The most common cause of noise is faulty cables, but if you can hear the hiss on the VMPro headphone out then it is possible that it is being generated within the unit. If you cannot hear the hiss in the headphones then see if the level of hiss on the line out varies with the output fader level - if not then the hiss is being induced into the signal after the VoiceMaster Pro in the signal chain.
Try using the mic, line and instrument inputs to see if the hiss occurs when using all three or just one of them. Also see if the level of the hiss varies with the input gain - if so then the hiss is being induced into the signal before the VoiceMaster Pro in the chain.
If you have tested all of the above and still suspect that the hiss is originating inside the VoiceMaster Pro then please contact us for further advice.