the car to power a home use amp

Actually, it contains 3rd, 5th, 7th, 9th etc harmonics!

Manufacturers never removed the harmonics simply because the UPS is designed to power up computer systems which use switchmode power supplies (which generally don't care if the input is sine or step-sine or square). Trying to use a UPS to power up any inductive load for extended periods of time is probably asking for trouble (or a smoked transformer... :) )

Don't get me wrong though - there are some UPS' that do in fact produce true sine-wave output - but these are significantly more expensive than the 'usual' stuff. I used to have a UPS manufactured by TI - it produced a really clean sine wave, but went dead some months ago.. sad...