If you have very soft water, then pH is really not a relevant measure. When soft water is added to hard water, the pH of the hard water prevails so there's no change to speak of. I'll need to know exactly the KH readings in both tap and tank water, and the frequency and percentage water changes involved, to be able to comment on KH.
I do 100% water changes daily using conditioned tap water and there's no adjustment at all. My tank conditions reflect the pH and KH of the area water table so life is very simple.