You can hear the preview/headphones audio into the master even if your volume faders are down.
If you are using software that creates a multichannel virtual headphones environment (like 7.1 or Dolby Surround headphones virtualization), this software may modify your Hercules DJ Controller sound-card behavior in WASAPI mode.
To avoid any 3rd parties audio software causing any interferences with your Hercules DJ controller driver, and also to be sure that DJUCED uses the best performance audio driver available, please download the latest ASIO drivers on the Hercules support site for your controller.
After you have installed the ASIO drivers, please go into DJUCED audio settings, and use the ASIO soundcard of your controller instead of the WASAPI.
It will solve the problem.