I just put a Xenium with OS2 flashed with M8 in a 1.6 Xbox.
After doing all the soldering using the pics for the 1.6 diagram provided by Team OzXodus it still wouldnt boot properly ( Got a red light but nothing else ) The guys in the OzXodus forum recommended soldering the D0 on the motherboard directly to a ground on the Motherboard, not the Xenium. Just did it a few hours ago and it worked Fine. Hope this helps for thoes of you still having problems.