I used the compressor/limiter on the 9000 some, as I now do on the 2000. I just use the factory presets and it works fine.
What you need to try is probably the Noise Gate. By activating it, you set the volume level necessary for the mic to pick up and reproduce sounds. By playing with the Noise Gate level, you can most likely eliminate the feedback problems.
The Compressor can actually ADD to the feedback problem, because it will try to amplify weak sounds enough for them to be heard. In other words, if you are not singing or speaking into the mic, the compressors "looks" for input and will attempt to raise the level of whatever it "hears".
Make sense?
DonM