I believe you're looking at the "Dialog Normalization" setting which defaults to -27 dB.
-18 to -20 is considered half acoustic level, so -30 would almost vanish.
This is a tutorial on using A-Pack, the grandpa to Compressor. I can't find anything on Compressor itself other than a DVD for sale on Ken Stone's web site.
[
www.kenstone.net]
Koz