I suspect, the mass reduction by alu, CCA wires (related to copper) well masked by mass of necessary membrane coating (glue, for example). Did you ever measure the mass of glue on membrane? At the Magnepans seems dominating part the glue and the Milloxan. Furthermore this layer make better mechanical coupling between wire and undriven foil sections, gives a better high-frequency response (paradoxically). Perhaps the copper can be enough good (against its higher specific mass)?