from what i know purple stems can be genetic or due to a lack of nutrients.
it seems to me that nutes you are using are pretty strong, espically if you soil is good and rich. if you over feed your plants the nutrient salts will build up in the soil and be deemed unusable by the plant.
my advice would be to leech the soil with straight water, wait a few days
and mix up a 1/2 strength foliar feed. use this till you see a change.