If you're using Traktor, there's no need to have the CDJ's connected to the Rane MP2015 via SPDIF or any audio cable. When used in Traktor, The CDJ's are acting as HID units only, essentially MIDI controllers–you're not using them to produce sound nor is any sound be sent to them; only data. They should be connected to your computer via USB or to a quality USB hub with external power. The mixer should be connected to directly to your computer via USB, and then select the MP2015 as the audio interface in Traktor's preferences.