Density is a special plugin, because it demonstrates things that ended up resonating through Airwindows plugins for years. It became part of the toolbox for implementing other things, and started in 2007 as one of the first Airwindows plugins. It’s the ultimate flexible saturation freebie.
Not just saturation, though! Yes, Density can be cranked up to incredibly distorted levels, producing a huge and fat sound. It’s got a type of distortion that is intensely thick and tubey, and it can be highpassed and then mixed in with dry signal to do lots of great things with the energy of the tone. But you can also set it to negative values, which gives you ANTIsaturation.
What’s that for? Not for thickening the sound and making it more upfront, which is what Density likes to do with positive settings. Rather, the opposite: it thins the sound and makes it more far away. This probably won’t seem useful immediately, but consider that you can throw it on reverb returns, on mix elements that have to sit back in the mix—upfrontness is relative, and you can work a mix from both directions.
Elements of Density are used throughout the Airwindows product line, but here’s where it started: one very flexible plugin, still useful today.