If you've covered the basic troubleshooting steps about the only way to pinpoint the problem part is to use known good parts for testing.
Xigmatek Dark Knight didn't have the 1366 bracket included? It's website shows it should have been there in the accessory pack. And the Users Manual shows both the 775 and 1366 back plates. Xigmatek Dark Knight
I searched for ''xigmatek dark knight s1283v'', and the only place I could find to buy it in the UK was here...which was handy, seeming as i bought a number of other items there.
But when it came, I found out it's actually the ''Xigmatek Dark Knight S1283'' - without the v, which doesnt contain the 1366 compatability. Stupid mistake to make, but never mind - only cost me an extra £10
As for testing the parts...I'll have to beg my parents to lend me their computer for a bit, seeming as my old pc blew up when my sister decided it needed a drink