Simple Q&A (By RAW) III

[Lord Lorac Silvanos]

A. 436

In my opinion, any DR possessed by the wearer of the item would not apply. The damage isn't from a weapon - it's from a magical effect.

I think you are confusing the attacker with the wearer.

The Amulet divides damage taken between the wearer and the attacker. If the wearer has DR that applies to the attack in question some of the damage that the wearer would normally take is ignored.
It is only the actual damage taken (after DR has been applied) that is divided between the attacker and wearer by the Amulet.

Or to answer directly; Damage Reduction is applied before Retribution.