If “the cube in the circle.” means the “cube within the sphere,” then it refers to the Enochian model of the cubical Universe within concentric spherical Aethyrs. If “circle” is literally meant, however, then we have an altar within a magical circle, and a phallic stone or cube existing within the feminine symbol. “Cube” and “stone,” of course, depict the object of the Great Work whereby “Father” and “Son” are seen as One (ABN). “Cube” also requires six sides, hence a solar (which also always means stellar) idea within the circle of Nuit, or (within the microcosm) of a given ego.
Correspondent Robert Westmoreland has written that “cube in the circle” suggests to him the Islamic Kaaba, “which is a cubic stone surrounded by an ever-moving circle of the faithful. The word kaaba comes from the Arabic word kab, meaning ‘cube.’” I am very excited about this perspective — it may even have been a specific image that inspired Crowley’s mind during the writing, and it integrates many of the other ideas expressed here.
In any case, this “axle” and “cube” are both something at the center, and the “wheel” and “circle” are both circular or cyclical. Both express Centrum in Trigono Centri, and many ideas we have aleady associated with Hadit, including the phallic, creative, central, vital, &c.