I'm not a chemist, so I don't know how well or completely I can explain, but...
Low EC/ppm, for the most part, indicates that there's not a lot of stuff in the water holding its pH steady and resisting changes to its pH.
(We're actually more interested in a property called alkalinity, which is the amount of buffering available to resist pH changes, but using EC/ppm here as a rough substitute.)
That means even a small amount of a strong acid/base (like your pH down/up), will change the pH very quickly. That also means when it's added to media with any significant amount of nutrients, those nutrients will govern what the final pH will be in the soil.
At least mid- to late-grow, I don't worry too much about my water-only pH since it won't have as much of an impact on the overall soil pH. I just control the pH of my feed.
When I do care more about my water-only pH (e.g., early grow, when there's not as much nutes stored in the media, I've been feeding at reduced strength, for certain medias, etc), I usually give myself a wider range for pH, use weaker acids/bases, etc.
pH up/down is intended to adjust the pH of higher ppm nutrient feeds, so they're usually a pretty high concentration (~20-40%) of a strong acid or base (e.g., phosphoric acid, citric acid, potassium carbonate, etc). You can try diluting those down to a lower concentration when adjusting water-only pH so that you're not adding as much of the acid/base with each drop. Or, you can try just using your normal nutes at a weaker strength.
Personally, I usually use ascorbic acid for ph down and a silica booster for pH up.
For hard tap water, the bigger concern is usually the amount of Ca in it (which typically ranges from ~25%-50% of its overall EC/ppm depending on the region) and balancing that with the K and Mg levels in the overall feed. For pH, that usually means it is more alkaline and has higher alkalinity (buffering to resist pH changes) than RO/low ppm tap. As far as adjusting its pH, I normally just worry about neutralizing the buffering - i.e., adding enough weak feed (or whatever I'm adding to adjust pH) so that the pH at least begins to change.
But, where I may try to control my feed pH to a target value +/- 0.2 pH, I'll only worry about getting water-only pH within about +/- 0.5 pH of that target as long as I've neutralized its alkalinity.
