I would not recommend augmentation of the bottom octave by EQ because distortion is terrible. If you increase power under horn cutoff to bring the deep bass up, then the distortion products are what are amplified by the horn. The horn doesn't amplify the fundamentals below cutoff, but it does amplify the distortion products.

So my conclusion is if you don't have the room to build a proper basshorn, don't build a truncated one. A properly tuned direct radiating sub is better than a basshorn run under cutoff and EQ'ed to compensate.