My comp was off each time the power was knocked out, but the X hasn't flinched from being fed whatever the APC gave it until I shut it off too.
Until the battery kicks in, a UPS will just be providing current from the utility power (with possibly some power conditioning depending on the UPS). The whole simulated/pure sinewave thing doesn't become an issue until the battery kicks in (unless you have an Online UPS, which are rare for home usage, and quite a bit more expensive).
the PFC circuitry takes care of sudden drops in voltage, and buffers the voltage so that the current is pulled in form of a sinewave. So, you don't need a pure sinewave
This directly contradicts what several people have experienced. UPS's that don't have pure sine wave output, typical have a stepped simulated sinewave output (instead of a smooth parabola, it's squarish). This can result in a state where there is a "zero output" state briefly from the UPS on battery power, and with some active PFC power supplies cause them to shutdown making the whole UPS utterly useless as a battery backup.
Also, there have been reports that this stepped simulated sinewave can cause the PSU to make a buzzing noise (a big no no for SPCR!). In extreme cases, some people say that a simulated sinewave can actually damage the PSU.
Cyberpower has a decent section on their website explaining the problem: http://www.cyberpowersystems.com/about- ... e-ups.html
You can't completely trust them though, since they are trying to sell you a specific product to solve the problem.
From what I've read, this issue doesn't effect all active PFC power supplies. But I can't find any reliable information one way or another about the Seasonic X-Series. Hence, I am asking for actual peoples experience with a UPS and the X-Series.