The Eaton 9PX is literally made for power factor corrected data centers, definitely not for a stereo. Hence the loud fan. You didn’t give any sources about how you know exactly where the UPS is used in the world. IT equipment is not necessarily that sensitive to distortion on the AC mains line.
Certain non-linear loads like a power amplifier or vacuum cleaner should certainly never be plugged into a UPS, unless you can somehow verify with measurements (risky and dangerous) that the peak current is within the 3:1 crest ratio. That also implies AC mains has much lower impedance and is therefore better for hi-fi equipment.
Not to mention that the UPS is not at all made to handle high inrush current, it is indeed made for servers to keep them on all day and every day. Otherwise you should use the UPS in high efficiency bypass mode by default, to let high inrush current pass through the UPS without damaging the inverter. The overload condition for a shutdown is 150% of the maximum wattage rating of the UPS. Confirmed with Eaton on e-mail. @tedsmith and @Paul also concur in this older thread:
Eaton Protection Box is however a series of surge protectors that are made for hi-fi equipment. But if the distortion or impedance of your AC mains is really miserable, then you should certainly be much more concerned about living another place than you should be concerned about using a UPS for hi-fi equipment…