Always, me and my friends thought that several goblin drums gives several courage benefits and minuses.

i.e. 3 goblin drums gives all good models -3 courage and the goblins + 3 courage.

Is this so, or is it just +/- 1 in all cases, and the drums other effect being the only reason (if any) to use more than one drum?

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Gold, silver, mithril.. You call that stuff treassure? Copper is the real deal!

The rule is not cumulative with other rules(looking in the khazad dum book next to me). So only -1courage to your enemy and +1 courage to your men.

I use two drums.
They are positioned at the left and right of my army.

Okies. *turns around and cheers loudly*

My friend is gonna hate it though, his goblins will have a hard time attacking my terrifying elves! *grins*

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Gold, silver, mithril.. You call that stuff treassure? Copper is the real deal!

The goblins should make use of their numeric overweight, the cave trolls and perhaps warg riders.
The drums are really handy against foes with terror.
Or when they get slaughtered really fast.