Hi Henry.
By building, listening and measuring, not only to my own designs and prototypes, but those of other designers also. Subsequent discussions with yet other audio designers and retired designers have suggested that we generally agree about the ranking.
Compared to machining, casting is cheap if large numbers of parts are to be produced, and therefore would be most attractive to a manufacturer such as myself. This is particularly true when the shapes get complex, which drives machining costs up, up, up. Unfortunately, so far I haven't been able to convince myself to use casting for anything more demanding than an active digital cable enclosure.
How many materials? Off the top of my head, aluminum(s), magnesium(s), brass(es), bronze(s), iron(s), copper(s), titanium(s), stainless steel(s). Probably more if I jog my memory.
BTW, none of the above necessarily means that you and I would come to the same conclusions. It very well could be that your audio system and listening habits are different enough from mine that we'd have to agree to disagree.
cheers, jonathan