I have a Yamaha EMX series powered mixer.... no problems. Get the most powerful you can (stereo 500 watts) even for backup, there is no substitute for clean power - better to run a 500 watt head at half power than a 250W at full........ and I would always recommend a stereo one over mono, any day. Modern arrangers beg to be used in stereo, in fact, many have sonic problems played in mono.

I prefer the bigger, mixer-like powered mixers. They have two independent FX units, and a very good tap-delay - great for tempo synching ballads and the like. But they are more expensive. Perhaps one as your main PA, and use what you have now as the backup...?