I've been reading through all the archives and I'm trying to understand exactly what the super baffle does. If I add one to my BIB will it negate the need for a BSC circuit or are they two different things?
It doesn't need either if it's correctly positioned in corners (which is what it's designed for).
A supra baffle lowers the step-loss frequency at / below which the acoustical load makes a transition from 2pi to 4pi radiation space. The frequency at which this transition occurs is dependant upon baffle width -the wider, the lower in frequency the step-loss point is.
In plainer English, above this frequency the speaker is projecting sound forward (more-or less), toward the listener, but below, it is radiating in an omni-directional fashion, so there's an apparant reduction in low frequency sound pressure levels at the listening position, as less energy is directed forward, but distributed in other directions.