The HX750 and 850 are Channel Well, a sort of customized model of the DSG platform. They are high effiency, but Channel Well opts out of making them truly silent, the 140mm fan starts at about 1000 RPM. While it's audible, it's not at all really intrusive, unless you are allergic to any kind of noise.
Now, the AX750 and 850 are based on the new Seasonic X based platform, and operate fanless at low loads. The AX series are also fully modular.
If I was buying anything today, I'd get the AX.