If you already removed or disabled all unnecessary hardware and enabled all the power-saving features available, then there isn't much left for any aftermarket software to do. In all likelihood, adding background processes will increase overall power consumption. For a system that is idle or under very light load most of the time, you may also benefit from using a small high efficiency PSU if you aren't already doing so as grossly over-sized PSUs tend to have poor efficiency at very light loads.
If you want power savings beyond that, putting it on standby while not in use would be the most power-efficient thing to do. Enable WoL on the server and use an app to remotely trigger it as needed.