The junior model didn’t work well with my APC SmartUPS SC 620. The UPS would shut down when switching to the batteries even at a load of 300 watts. The other two models could work with my UPS at loads up to 340 W when powered by the batteries and over 400 W when powered by the mains.
These PSUs are not very confident working with near-zero loads (take note of that part of the diagrams built for the X-560 and X-660) but have no stability related issues otherwise. None of the voltages violates the permissible limits even at highly misbalanced loads, i.e. when there is near-maximum load on one rail and near-zero load on another rail.
These PSUs have virtually no output voltage ripple. People often ask me what power supply is best for a computer which must deliver high audio quality via analog output without an external DAC. Although I don't think that high-frequency voltage ripple of 20 millivolts more or less can affect the sound card (the mainboard is going to have a stronger effect on the latter, for example), you may want to consider these Seasonic products for that purpose.
The fans did not rotate at all at loads up to 150 watts and remained silent for quite a long time after that threshold. It is impossible to hear a high-quality 120mm fan rotating at less than 600 RPM from inside a system case. The Seasonic X-Gold PSUs become somewhat audible at loads over 500 watts but the X-560 does not really get noisy at all. The X-660 and X-760 can be called noisy at loads of 600 watts and higher. That's just an excellent performance.
Compared to the previously tested X-750, the X-760 is somewhat quieter at loads above 400 watts. This may be due to the larger heatsinks of the X-760 or to random variation in their component specs. By the way, the X-760 uses a 2-pin fan connection whereas the X-750 has a 4-pin one.
The efficiency is expectedly high. Each model is over 90% efficient, the 760W model being the only one to touch the 80% mark with one end of its graph. Each of them is 90% or more efficient at full load.
The standby source is okay, too. At the maximum load of 3 amperes its voltage was never lower than 4.9 volts, the allowable minimum being 4.75 volts.
Summing up this part of the review, I can say that Seasonic has come up with excellent power supplies which are quiet, stable and reliable. It’s not clear what reasons the company had for producing the second revision of the X-Gold series. Although there are conspicuous changes, none of them is critical, and I haven’t heard about any problems with the first revision. Seasonic seems to have just corrected any small shortcomings that had been noticed over the previous year and made the good power supply even better.