when you need to apply a discount at the line item level, it is time to use the eCart calculations tab.

if the breaks will all be the same, i.e 1-9, 10-49 etc, you will just need a column for each discount amount (you can get really crazy though and set the quantity breaks on an item by item basis by adding a low high column for each quantity level).

For this example, lets say the discount breaks are 1-9, 10-49, 50-100. you would need to add 4 new columns to the cart discAmountA - this is the amount of discount for 1-9, discAmountB - this is the amount of discount for 10 - 49 etc...

you can then create a new calculation of:
name: itemDiscount

(abs([DiscountGroup] == 'A' && ([Quantity] >= '1' && [Quantity] <= '9'))*[discAmountA]) + (abs([DiscountGroup] == 'A' && ([Quantity] >= '10' && [Quantity] <= '49'))*[discAmountB]) + (abs([DiscountGroup] == 'A' && ([Quantity] >= '50' && [Quantity] <= '100'))*[discAmountC]) * [Quantity]

then on the discount tab, set the discount to be subtotal of column 'itemDiscount'