First, if you want to get an accurate reading of the soil pH using that method, you must use deionized water. Tap water at 8.0 is that high because of dissolved minerals, which buffer the pH and will throw off your reading significantly. What you are seeing is not the pH of the medium, but the pH of the solution after the medium and your tap water have interacted chemically at ambient temperature and pressure for one hour.

If you water with 6.5 and the runoff comes out lower, your medium is causing that drop.

The medium pH and nutrient pH are only independent until they come in contact with each other. Once this occurs, whatever compounds are causing the pH of each to read as it does will interact and give you a pH that reflects EVERYTHING in the new solution- source water, dissolved atmospheric gases, medium, and fertilizer.

Coco and ProMix are both soilless and should be run about ~0.4 pH points lower than the soil range. Coco also scavenges calcium and may contain high levels of salts and potassium. Promix comes factory buffered with lime, which works well for 6-10 weeks depending on your water's pH and hardness, and if you are adding a fertilizer or supplement such as humic acid or silica blast that buffers pH.