ph for hydro is 5.8 and the ph for soil is 6.5. Some people run promix like a soiless because it has no minerals and shit in there. They also use hydro nutes to make sure it gets the full line. I fortify my pro-mix with blood, bone, and kelp and composts and worm castings and some other goodies. It is like a super soil. Anyway, I am having the same yeild probs as you. I dont think that the ph is the prob. It might be the strains. IDK, I also in a tent with a 600 watt lumatek, except tring to grow trees. I will try the lower ph anyway as I have had that prob in the past. Only it was very noticable in the leaves. One water fixed it in a few days with lower ph. Serious rebound as the plant began to use N again in veg. I have the bud prob. lol. ARRRRR.