A UPS imposes a limit on the amount of current available to the connected equipment whether operating from the battery or from wall power. Power amps can easily draw large amounts of current, thereby running into the limit of the UPS. Good surge protection is highly advisable, but I'd recommend not using a computer UPS for this task - products from companies like TrippLite, Panamax, or Brickwall would be a better choice for protection of home theater equipment. If you do have serious electrical storms in your area, you could even consider surge protection at the main panel. Such equipment is reasonably priced, although it's best to get a licensed electrician to install it for you.