I could read the manuals but I'm feeling lazy today. Who has used both and can shed light on the strengths and weaknesses of each?
I've used MaxxBass and RenBass for years and have been using LoAir for about a year. Here are my observations:
MaxxBass and RenBass have a similar sound. RenBass has simpler (slimmed down) controls. While they sound the same, they also can sound different :) I use them for adding punch, thump, or weight to a sound. Sometimes MaxxBass works sometimes RenBass, it all depends on the material.
LoAir is completely different. It is supposed to be like a subharmonic synth (I've never used one). I use LoAir when I want boom, when I want to shake the house, and when I want my sub bumping. While I find LoAir is much more of a "wow" effect than the others, I also find that it is easier to make things sound terrible using it.
When I'm designing guns, I rarely use LoAir but often use MaxxBass or RenBass. LoAir works better for me on things like explosions or big monster footsteps/bodyfalls. I also tend to use LoAir at more extreme settings than the other 2.
That's my take on it, others may have different experiences.
As I understand it, LoAir enhance/adds in subharmonic or low end content. MaxxBass adds in harmonic content in the upper registers to help perceive the low end of a track as more present and louder witout overdriving the low end (it's really good for that too).
So MaxxBass=adds/enhances upper registry harmonics for low end sound, LoAir=adds/enhances lower frequency content for higher end/thin sounds.