IME, not only the choice of material, but how the material is processed has a significant effect on the sound.
Again IME, the descending order of sonic quality is: 1 machined from solid billet, 2 extruded into approximate shape then machined, 3 forged, 4 vacuum or pressure-cast, 5 sand cast.
I should also point out that the phosphor bronze alloys used in machining are different from those used for casting (higher tin and lead content).
hth, jonathan carr