What Liofa said, Aggressive Horn all the way no exception. Its increase in damage for the entire group is great and the extra Health you give out also helps in survivability.

BUT you were onto something with Replenishing Barrier. If you slot it on your front bar and warhorn on your back bar you can gain a lot cause of the Support Passive Magicka Aid. Rank 2 of that passive gives you 10% more magicka recovery for each Support ability slotted and the ultimate counts. Now you don't actually want to use the Barrier but you want to have it slotted so that you can gain magicka faster which will as a DK help you use your utility skills much more frequently and if you want stamina back being able to spam igneous shield more often is the way to go.