Hey fatz... ill see if i can help you out. First of all you are right I would steer away from any chemical PH controls. All these are is a temporary fix and a small mistake can be a disaster.
6.6 PH isn't something to fret over first of all. A stable PH is much more important than an ideal one. What I suggest you do is leave a cup of tap water out over night and test the PH of the water you keep doing water changes with. This may help explain this sudden drop. Water coming out of your faucet may be very soft, which means that its ability to buffer PH due to outside influences (CO2
among others) is very very low. There are ways to raise your PH more naturally like using crushed coral in your filter if its really important for you to bring it up. A KH
test kit will help us to analyize how soft your water is, and may lead to the cause.
As far as the browning leaves go, what size tank is it, what kind of substrate, do you use fertalizers and what type of lighting and watts. This information is important to understand if your plants have everything they need to flourish.
As far as reducing nitrates by doing water changes, this isn't always a bad thing. Sword plants are nutrient hogs, and even with a pretty heavy bio-load of fish in the tank, a growing sword may still eat through nitrates faster than the fish naturally produce it. I would keep up on water changes as normal and possible consider dosing nitrate artifically. There are some good stickies in the planted tank forum on dosing. HTH