I think this is one point where we should integrate with deltaslim's psychoacoustics thread at the GC forums.
At the sake of opening a can of worms, we might even have to take Fletcher-Munson into consideration, but I feel we must forsake this to prevent further nosebleeds, aside from other things. Another thing that can increase apparent loudness is the use of enhancers such as those found in Aphex, BBE and other aural enhancers. These work on harmonics, btw, although I tend to favor the Peavey Kosmos. Unfortunately, my preference is based mostly on user reports and not on actual hearing tests since I can't find any Peavey units around here.
One plugin that I tend to favor is the use of multiband compressors. The Sonitus multiband in Sonar gets a lot of use in my mixes, especially when patched across the drum group channel.