Current powder formulations are dense, compact
and so you use a much smaller dose,yes, you are using much less water. However powders unlike liquids depend heavily on buffering agents to soften the water so the surfactant can dissolve into the water and go to work. If the balance between hardness and softness is not just right then hard water will form solids with the builder/softener and also the surfactant used.
It is important to keep the water softened throughout the wash.
Also all soils in laundry drive up the acidity of the wash water, soils are acidic by nature. Clearly a regular dose of TIDE HE powder in your water is not enough to keep the wash liquor alkaline through the whole wash cycle. Since you have such hard water you need a booster in there. You can add STPP like you say but you can also add "washing soda" to keep the alkalinity up and plain old washing soda is a lot cheaper and better for the enviroment.
That is why I am formulating "SPLASH" to have a powerful, compact alkaline builder for hard water situations such as these.
In one recent batch I put too much builder in and got cotton clothes that came out of my Neptune looking like they had been buffed upagainst a sand wheel! So you can have too much alkalinity as well as too much acid. Alkalinity of around 11-14 is a good zone to be in for wash water.
Great Job on the Neptune, I love my 10 year old baby, but I also love my American made Speed Queen too!