I'm using an APC Matrix 5Kva unit. I've also used a APC Symmetra 5Kva unit and a much cheaper SmartUPS 1500. I've been very happy with them all as I have the same situation with dodgy main power in the house (an older house in the woods).

You should be able to get a SmartUPS 1500 for about $200. Depending on your power requirements, you could get two and use one solely for the amp.

Cheers,