i have been growing now for 18 years and have used diffrent grow mediums,at the moment i am soil less.and have messed with ph a great deal looking for better results.in my experience 5.5-6.2 is fine not below this and not over this,certain strains like different phs but i always go as close to 5.5 as i can and have gained better results from this.
good luck mate i hope this helps