On my audiobahn A8000T I have the bass boost all the way up, then I set the gain...but, does the bass boost just make it louder around a certain frequency? Like 40-45 hz?
Yes, it just boosts lower frequency's, but if you have a good system to begin with, you shouldn't need it. Setting the gain shouldn't have anything to do with your bass boost.
I have 3 Audiobahns (A8000T, A4002T & A2002T). I was unhappy with the SQ while using the sound processing features on it (i.e., the x-over, bass boost, etc.) I've since then bought a Clarion 3-Way x-over and use it's x-over and bass boost/gain functionality. IMO, it sounds much better. Personally, I wouldn't use the bass boost on the Audiobahn, but that's just from my experience.