Experts say that the best water for making coffee must contain around 150 ppm of total dissolved solids (TDS). Okay, cool. Now how do you achieve that?
This is just a rule of thumb; opinions vary as to the ideal water quality. Some coffee shops use reverse osmosis for their water, but the fineness of the membrane may decrease the amount of solids in the water, resulting in an overpowered taste. Then again, too many solids will interfere with the organics in coffee, resulting in a flat taste.
Reverse osmosis retains around 10 ppm of TDS, which is still 140 ppm less of the acceptable amount. That's why it must work with another water treatment system: softeners. They're not directly involved in reducing TDS since they don't do that. Instead, they remove the ions that make mineralized water salty.
However, mixing water that underwent reverse osmosis and was treated with softeners can help achieve the ideal TDS for coffee. Tap water contains an average of 350 ppm of TDS, so start your computations there. A mix of 40 percent softened tap water and 60 percent reverse osmosis water is probably as close as you'll get to the 150 ppm range.
Nevertheless, nobody likes unfiltered water in their cup of joe.