Computer UPS systems are not good for audio components because most of them produce square wave AC when on battery backup, as opposed to low distortion sine waves normally present on the AC powerlines.
The square waves are highly distorted and have a lot of high frequency grabage you do not want being fed into your analog power amps and preamps. While your input transformers will filter some of this crap out, it may also cause them to run hot or make noise.
Also, because the equivalent RMS value of a square wave has a lower peak voltage, the voltage level of the rectified waveforms will be lower, resulting in a lower filtered voltage in your power amp or pre-amp. A true square wave will have only 70% of the peak voltage of a sine wave. Which means a power supply of 50V (on AC sinewave) will only be 35V with a square wave. This will create obvious operating problems in your amplifiers.
There are some products that produce true low distortion AC (like the afore mentioned PS Audio products), but brands like APC do not. Edit: APC may make true AC sinwave generators but they are for larger industrial applications and are very expensive and not suited for in home use.
