Apple today released Mac Pro SMC Update 2.0, enabling a Mac Pro to enter Power Nap mode without running the fan for Power Nap activities



From MacRumors:

The update also includes a fix for an issue that caused a low-speed USB device not to be detected at startup. This update is recommended for all Mac Pro (Late 2013) models. This update enables Mac Pro to enter Power Nap without running the fan for most Power Nap activities, and addresses a rare issue where a low-speed USB device may not be detected at boot.

  Apple Releases Firmware Update for 2013 Mac Pro to Fix Power Nap Issues