Working with my APC SmartUPS SC 620 (non-sinusoidal output voltage), the 650W Aurum CM was stable at loads up to 404 watts (when powered by the mains) and 340 watts (when powered by the batteries). These numbers were 390 and 350 watts, respectively, for the 750W model. The PSU and UPS pair was blameless until these loads but the UPS would shut down, reporting overload, as soon as the load got higher.
The 650W unit is let down by the +5V rail whose voltage sags below the permissible level of 4.75 volts at high loads. On the other hand, no modern computer can produce such loads (it is the +12V rail that has the highest load whereas the 5V rail accounts for but 30-40 watts), so I don't find any critical problems with the AU-650M.
The AU-750M is somewhat worse. Besides the +5V voltage sagging at high loads, its +12V voltage goes below the permissible minimum when there is a high load on the +12V rail (over 600 watts) and a low load on the +5V and +3.3V rails (less than 40 watts combined). I don't think that such greatly misbalanced loads can be found in a real-life computer, yet most other PSUs cope with them. So, although this is no critical problem, it is a downside nonetheless.
The high-frequency output voltage ripple is quite conspicuous at full load but its level is within the permissible limits.
The same goes for the low-frequency ripple.
The PSUs are both highly efficient: over 92% at the peak and no less than 89% at full load.
Unfortunately, the fan regulation of the Aurum CM units is the same as in the Aurum. When the PSU is turned on, its fan starts out at 800 RPM but quickly accelerates to 1200-1300 RPM even at low loads. After that, its speed grows up linearly along with the load (i.e. the temperature inside the PSU) until it hits the maximum of 2230 RPM.
Thus, the FSP Aurum and Aurum CM power supplies are only average in terms of noisiness. They are not quiet, let alone silent, even at low loads (like 50 watts), so they can only be recommended for office use or for people who don’t care much about how much noise their home computer makes. In fact, nearly all of the competing products from other brands are superior to the Aurum series in terms of acoustic comfort at loads up to 500 watts and many of them are better at higher loads, too.
The standby source is stable and delivers its promised 3.5 amperes. As a matter of fact, I can’t remember more than three PSU models priced higher than $30 that couldn’t pass this test.