I wouldnt use anything but Promix, its a tried and tested medium. I have been using it since I can remember and I have never once had an issue.
You have to remember any soil that has any amendments within it already is susceptible to inconsistencies, due to human error, machine error, wrong mixtures, etc...
I'd rather start with a base and add amendments to it myself if necessary, atleast then you know exactly whats it, and how much, and its properly mixed by your own hands.