As alluded to here, softness is not the problem. Its the inadequate alkalinity to balance the increased acidity of dark beer grists. That water is damn near the profile that my RO system puts out. If that's your tap water, my hat's off to you. That is a perfect palate to craft great brewing water. You will definitely need to have the capability to add alkalinity to your water if you're brewing anything but light colored beers.
Alkalinity is easy to add and a little more difficult to remove. Chalk is a so-so option for adding alkalinity since it needs to be dissolved prior to brewing. There are work arounds that mention using twice as much chalk as required since the evidence is that it only provides about half its calculated alkalinity. I think that EZ Water might assume this in their calculation. Bru'n Water calculates the full alkalinity that chalk will deliver if its fully dissolved. Palmer's spreadsheet also calculates the full alkalinity produced by the chalk addition. In using either of these programs, you just add twice as much chalk as calculated to try and provide the intended amount of alkalinity.
A better way of adding alkalinity is to use pickling lime since it dissolves totally in water. To my knowledge, only Bru'n Water includes the calculations for using lime. It will absolutely add the indicated alkalinity. Its very strong though and you should only add it to the mash since it will bump the pH of the mash water way up before you add the grain.
Baking soda is another option and it works well, but you have to avoid adding too much sodium to the water.
Be careful with any mention of your starting water pH. It means nothing. Just because the water has somewhat high pH, the lack of alkalinity will cause the pH to drop like a rock when any acid is added to the water.
Its interesting that 5.2 Stabilizer is mentioned. As AJ DeLange has mentioned, it doesn't work as intended. But in the case of this brewer's low alkalinity water, it will keep the pH from dropping too low. They should really call that stuff, 5.8 Stabilizer since that's the pH it will produce. That's too high for good brewing, but it is workable. I still recommend that you learn to properly adjust your brewing water with a good program like Bru'n Water.
. I did find that once I went all-grain, I needed to add a bit of Calcium (Gypsum) in order to get decent conversion - and of course, some other stuff for darker beers...
