I believe the majority of HX power supplies are built by CWT. The HX750, HX850, HX1000 and HX1050 units are all CWT, while only the HX520, HX620 and HX650 are Seasonic.
Something else needs to be said about the respective strengths of manufacturers; they alter their designs according to what the brands want, in terms of performance, component selection, build quality and price.
So, for instance, there are CWT designs which are higher quality than Seasonic designs, or implementations thereof, and vica versa.