I don't think it's the lighting either. i run a 48" NEP on my 75 with 4 Blue +, 1 AB Special, and 1 KZ Fiji Purple and it's plenty of light for my SPS. Also, that stock 10,000K Current bulb puts out less PAR than the ATI Blue +.
Nutrients being too low was one of my first thoughts, or chemical warfare from the softies.
If it were me, I'd replace the Blue +, and bump up the feeding, and try to increase the nutrient levels in the tank (not too much of course), and see how that goes. If that didn't work I'd consider increasing the amount of GAC, and possibly look to add a reactor for the GAC & GFO.