I find this all to be kind of ironic. You have perfect water, from the tap, to utilize aquasoil to its full extent seeing as how it's a buffering substrate, yet you're fighting it. Why fight it? Are you going to stock animals that cannot handle low pH?
KH is what fish depend on for osmotic regulation, even if the pH from your tap is 7.4 and your tank is 6.4, with both sources being 0 dKH, you won't have any issues. The water you add will be buffered down and stabilized rather quickly. Plus the acidic environment will greatly benefit most plants that we grow in our aquariums (not to say alkaline conditions can't grow amazing plants of the same species).
If it's for a betta, they can do very well in a lower pH.
With a buffering substrate such as aquasoil, you are going to be fighting it to create a balance and that will be more deadly to your fish than going with what it naturally wants to do, unless you can completely exhaust its buffering properties before adding livestock. The constant adding of KH, then subsequent eating of the KH by the substrate, then adding KH, etc etc is going to be more detrimental than just going with the flow.
Just my $0.02