Since Curved Horn's damage increase counts as flat damage, its damage up is applied after every calculation, but before damage multipliers. As a result, it is not subject to the lower effect in damage that other damage-boosting items are subject to. Other flat damage increasing effects, such as those granted by Death's List or Void, share this rule.
Curved Horn is still affected by damage multipliers, both beneficial ones (such as Cricket's Head or Eve's Mascara) and detrimental ones (such as 20/20 or Soy Milk).
This excludes a character's innate damage multiplier (for example, Azazel's 1.5x damage multiplier does not affect Curved Horn).
Death's List or 
Void, share this rule.
Cricket's Head or 
Eve's Mascara) and detrimental ones (such as 
20/20 or 
Soy Milk).
Azazel's 1.5x damage multiplier does not affect Curved Horn).
Judas Unlocks 
The Lamb Unlocks 
The Binding of Isaac: Rebirth
