I think customers are happier from the aspect of the Discount Coupon is not valid on Products with other discounts than they would be to lose the quantity discounts to get the Discount Coupon ...

Trying to take away the Quantity Discounts when a Discount Coupon is used makes it feel like they are getting "ripped off" even though they are not ... but it is a mentality of "I saw Discounts for buy this many and then you "took" them from me!" ... and where many might not notice this happening until *after* completing the order, this might save some grief ...

Let us know how this works out for you ...