Your Seasonic x-660 looks like you have good taste in PSU's, and I agree, quality is formost, but sizing... You never want to exceed 70-80% load. Thats when it really can damage a PSU, if its constant usage. The closest I have run is 600 watt load on an 850. That even worries me a little. So, get a kill-a-watt. See your wall load. Then multiply by about 80% to see your max capable(efficiency), then multiply that by 1.3 or so to see the optimum size you should get.
So in my case, a 600 watt drain and an 850 watt psu, is at 88% load, and thats really pushing it. That 850 only gives about 680 watts to the computer, and with a 600 watt drain, thats asking for trouble.
Edit. Its possible that the PSU manufacturers rate their PSU's after loss, so don't yell at me if I got this wrong.