A great read if you are interested in why cast iron effects the taste of water.
It is amazing how different the mouthfeel and flavor of the water is when boiled in a Tetsubin (cast iron kettle). The water has a sweeter taste when drank alone and typically intensifies the aroma of the brewed tea. They are not cheap and can be finicky to care for, but if you demand the best tasting tea I’d recommend considering one.
New York + Japan's Finest Sencha, Matcha, and Gyokuro