Bard Instruments

[Valli]

Hello,

Just a quick question about the bard instruments and their effects.
I recently purchased a McVaxius' Horn of War, and noticed it did not increase the amount of damage of my brass modifier songs.

My question is: What benefit's do the higher modifier instruments provide?

Do they decrease the chance to be resisted?
Is the Bard instrument system bugged?

Thanks for any help offered =)