Just out of curiosity, what are you trying to bring it down to? I ask because I have to brew with bottled water (my well water pH is around 4.6), which usually has a pH that hovers around 7 +/- a few points. My mash pH is usually dead on from just the grains effect on pH. I may add some brewing salts to my mash to help drop it or raise it a bit if i feel i need it, or if I'm looking for a desired effect. But more often than not, it hasn't been necessary.