Try Yoshihiro's M8 beta4 BIOS.. I've found I have to use it on most of the v1.6's I do.
Easy way to test if the box isn't working. Turn it on, off, on, off, etc, etc.. I've found if the box is going to fail, it'll start black screening after the first shot, sometimes second. Some 1.6's work fine with M8+ (preflashed on the chips), but most need Yoshi's to work fine.