I might guess that you have been reading the right places and found that the PH down is pretty much "snake oil" as it only lowers the PH for a short time and that leaves your fish suffering through trying to match/adapt to a changing target? Often when you have PH that high, you will also have very high buffering and that makes it hard to lower the PH.
Stable is far more important. I find there are many advantages to treating water before it goes in the tank rather than after if you are speaking of changing the basic parameters like PH/GH/KH. If speaking of treating to remove chlorine or chloramine, most feel there is only some difference in doing it before or after. If using dechlor, you can save some money by adding it before the water goes in the tank as you treat only for that amount and if you wait till after , you need to treat for the whole tank volume which costs a bit more.
I like treating my water in a barrel and then letting it set to come up to tank temperature so that there is not a shock to the fish.