Bass guitars can produce low frequency sound that will some times want to distort your PA rig. If you are using heavy eq to get the sound you need this will help. Try flattening out the eq strip a little, and start adjusting your compressor. This will fatten up the sound and level off the peaks and valleys making the bass easier to bring up in the mix. I prefer not to use a lot of compression on instruments unless it is absolutely necessary.
Vocals are a different story.
Singer's voices fluctuate continuously and compressors can even this out. If the singer then hits a screamer, the gain reduction is already happening, but does not give you the pumping or breathing sound. Attack time is fairly quick. Release times a little slower. Putting compressors on inconsistent singers will make your life easier and their sound much better.