Seasonic is the OEM (Original Equipment Manufacturer) for all XFX PSUs and is basically considered one of the top PSU makers. The Antec HCG series also comes from Seasonic.
EVGAs older models are not great but the recent G2 series models are made using Superflower models and are really good quality, especially for the price. I haven't done much research on Silverstone so I won't comment on them.
Basically, when you say you trust XFX, you are saying you trust Seasonic's parts/build quality.
For PSUs, it is usually more important to see who OEM is since many companies use multiple suppliers with varying quality (like corsair who use 3-4 different suppliers, some great and some bad). The reason most ppl trust XFX blindly is because they use Seasonic exclusively