Are you saying to ph the water to 6.5 but don't worry about soil(runoff) ph?
I'm saying that is what I would do... see how the plants take to it and go from there.
I got around 22 grams each from 2 plants and 10 and 14 grams from 2 others in the same grow so 4 plants that yielded less then 3 oz. My current grow has 3 and they might get a half oz each.
I just looked in on your grow. It looks like it is possible your growing conditions are not ideal and the plants in all those pics look over-watered... it may just be the pics.
The pheno's you have (they look like Kush) are not going to yield like bushy pheno's will, so that is part of it.
If you don't let your soil dry out between waterings, your plants won't have ideal root growth.
I couldn't see what kind of lighting you use.
Bottom line, things I see that could be affecting your yield: Grow op that isn't ideal (lighting, ventilation, temps. etc... any/all of these can affect yield.
Over-watering, or poorly drained soil-mix (I didn't see much perlite in there) can also affect yield since it can drastically affect growth.
Pheno... I usually get a pretty good yield but, I have grown some pheno's that yielded about 15 grams in a 3 gal pot. I usually get 30 or so under the same conditions with normal pheno's.
I hope that helps. X