I have never liked the sound of a PA as a monitor. The PA speakers are just not accurate, they are made for pumping sound at high volume, not reproducing with sonic clarity. Studios use combinations of super quiet amplification and ideally accurate speakers for listening, because they are listening to a true sound that can be well represented on any stereo. I would suggest this, however, i f you have to use PA speakers...
1. Know that there isn't alot you can do if your amp is noisy (and most PA amps usually are
2. Mix your song, go listen to it on a stereo you are familiar with and then make notes. You will be able to see what you need to adjust, then go remix the song.(I am very anal about my car stereo, and I know what sounds good and what was mixed well when I listen there)
3. Try a bunch of different stereos, just to make sure you really like your final mix.
This is more time consuming than just taping or burning a cd and flying with it, but you will get way better results.
Pete