I've always used tap water for my brewing and I feel like I have a decent process worked out. Still, I wasn't getting quite the malty flavors in my malt-forward brews. The hoppy brews came out fine. The single analysis I have for my municipal water shows 90ppm sulfate, not overly high but I've come to suspect it might be the source of a certain sourness and the lack of malty flavor. Plus I don't know how variable it is but the source water is the Mississippi river.
In any case I've started using RO water and building from scratch. The first attempt was a hefeweizen recipe that was coming out pretty well but a little tart. Just had the first bottle from the new batch and I'm really pleased, it has more grainy flavor and none of the tartness. I have a best bitter kegged and chiling and just bottled a dubbel, so I'll have more evidence for or against this hypothesis in the coming days. I'm hopeful, and frankly excited since this has been a thorn in my side for some time now. Plus I found out Walmart sells 5gal of RO for $1.65 so its not even a huge expense like I had thought.