Topic: neutral henna/amla for dark blonde hair

hi, i have dark to medium ash blonde hair (natural belayage) . i have bleach damage on the ends from hairdresser which i will cut off next lengthening date in august. the rest of my hair is not very damaged but i used a H20 hair colour thinking it wouldn't do any harm on my hair as it said it doesn't change the structure of hair. but my hair turned brassy, so if you ask me it does change the structure. anyway, i am trying to grow it all out virgin but wish to cover or tone down the brassy colour...what do you recommend i start with? do i begin with just neutral henna, do i add amla to it or do i use the pale blonde henna and add amla to that or just use the pale blonde alone? will this tone down the orange? i would like to get it back to ash but it doesn't seem possible in anyway with cassia, so a gentle golden colour will be better than harsh brass.
please help as i don't know which way to go.