You would probably need some XLR to RCA converters but those shouldn't be terribly pricey. You would most likely only get half the power out of the car amp as its power is rated at 4 ohm rather than the 8 (or worst case 16) ohm that most PA speakers present. Aside from that, I don't see an issue with trying it. One thing to consider would be buying old Pro amps off ebay. Mackie 1400, Crown PS-400, and various other Crown, Crest, QSC and a host of others can be had for fairly cheap and they tend to last forever.
