Your settings seem fine. Disregard the Ext. In settings, that deals with hooking up a multi-channel device such as a SACD or Blu-ray through analog cables. When using Ext. In, the sub needs atleast a 10db boost hence the recommended 15db. These settings have nothing to do with your other subwoofer settings, they are completely separate and are not the issue.
I would recommend trying attaching a 3 prong to 2 prong adapter to the power cord on the EP800 and see if those strange sounds go away. You can pick them up at a hardware store for a couple of bucks. They look like this (link below). This is not a permanent fix if it works because it defeats the ground but it is worth trying as a troubleshooting measure.
http://www.enjoythemusic.com/SuperiorAudio/equipment/0107/plug.jpg
