Well water. Going through natural Earth processes allows water to develop mineral concentrations, and other great stuff.
This in itself can help because it is energy. Better water will give the body more natural energy than tap.
I use a good filter that I run tap water into, but it isn't the same as mineral water. It tastes just purely neutral. Pure, mineral water however has noticeable flavour.
An alkalinized body is healthy. Yerba Mate's chlorogenic acids, are strong anti-oxidants, destroy free radicals, and restores vitality. When Mate is consumed continously the body becomes highly adaptive, and immune, alkanalized. Drinking good water alone is not enough. Good diet too.
If I am to guess, it makes sense to me that tap water would cause less extraction based on volume minimally. Pure water is not filled with 100 chemicals, and parasites thus more room for Kavalactones, but all this would do is create a less dense Kava per amount of KL's squeezed out. The hardness of tap water is already insanely dense, and greasy.
Essentially I said smooth water, less dense, more smooth Kava.