Pale yellow hair to beige ash blonde

I have pale yellow hair extensions, and I was going to counteract the yellow with violet.  I don't like warmth in the hair, I prefer cool ashes. I would like a base of 9. So can I just throw on a 9V?

Is anyone familiar with Belevance. They have the 100 series. 100 V, Would this be an appropriate toner for pale yellow hair. 

Does it require a filler? And how do you mix filler?

I don't want to have the colour wash out (demi) so is it safe to use 10 vol?

And the other extensions have an orange undertone, too much warmth. I was going to counteract with blue (ash). I'm thinking of just using a toner, to keep the same level (an ash toner.) But for the pale yellow hair, since I want it significantly darker, how do I do that?

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