As a nitpick: You always know the filter transfer function, it's the DFT of the impulse response (and without the impulse response, you obviously cannot convolve).
> generating a filter curve for a 300Hz slope, both channels were divided (A/B) against that curve
as defining a parametric EQ band. If you just generally want to invert a measured impulse response, then yeah, you don't know the underlying transfer function.
Since these operations sort-of commute, it is easier to understand if you invert the order of operations:
1. Invert the spectrum (ideally through something less sensitive to noise than what's done here). Now you have an impulse response that will, in theory, give you a perfectly flat frequency response.
2. Apply a 300 Hz slope. Now you gave up some of that flatness, but you have something that's physically realizable without murdering your speakers.