I would add something outside of the build factor - metal sounds invariably involve a LOT of compression which can muscle out the natural sounds of the instrument more than say, a overdrive blues tone or a clean tone. Basses are more than as not played relatively clean so the character of the instrument shines. I say invariably, because I prefer to back off the distortion to regain tone and dynamic!
Also, it depends on what style is being played. Big long drawn out distorted chords? Staccato picked phrased? Solos? All have their different flavours, and to a degree, styles of instruments which will suit them better. I like my EXPs for big sounds, big chords etc. I prefer my Ibanez S for solo and slower chunky characterful rhythm. I prefer my EXPs for low grindy chunk. The Ibanez is a bolt-on. One of my EXPs is also bolt-on, and the other is a set neck. I used to have a Jackson neck-through Rhoads V with a Floyd and I felt it was too biting....go figure! According to the bubblegum cards, it should have a big sound eh? Not so....there are always exceptions to the obvious....
Searching for the "ultimate metal sounding guitars" is like searching for the blackest black - there's more character and room for tone-searching to be found this side of the black-void-staring fence :-D