The Waldorf, designed by Richard Long, was contemporary to the 4520, but a very different beast and intended for the dancefloor, while the Jensen Imperial was designed for the lounge. The Waldorf was inspired by the Jensen and is supposed to have a rounder bass than the 4520, most likely because of the bigger rear chamber. The simpler horn path also leads to less reflections and nulls, which will probably affect efficiency and linearity. The 4520 was 4ft tall while the Waldorf had an extra foot (30cm). And for the sake of improving the lower spectrum, nothing can keep a builder from making it 6ft tall.
The 4520 design can easily be modified to reach 27Hz in horn length, but not in horn mouth.
But getting back to the original subject of this thead, I suggest the OP to look at horns if high SPL is the goal. It will require to build many, many 4645 to get the impact of a scoop.