Most speakers are rated at 8 ohms. If this doesn't sound important, please note that a 600 watt power amp may only perform at 125 watts when two 8 ohm speakers are plugged into it. What bridged means is that you take the two stereo signals and join them together so that the power will only support one speaker. You will need two power amps to support two stereo speakers if you bridge the power amp. The next important part of the PA system is the speakers.
Speakers are more important than a mixing board.
Remember, you can also plug two mics directly into the back of the power amp and not have a mixing board at all. Look for speakers that have lower ohm ratings. Be careful to check the power wattage of the speakers. If your speakers are not rated at the same or more wattage than your power amp, you can blow the speakers. Placing the PA speaker on a stand during a performance will also make them sound louder.