I have a creature, Wild Beastmaster, that increases all other creatures I control whenever it attacks (I assume that means the attack triggers this). However, since a card like Trumpet Blast only affects attackers that are already declared (as per the rulings at the bottom of that page), it only works if attackers have already been declared. Would the Trumpet Blast be able to effectively be doubled through the Wild Beastmaster, or would the specification of "attacking creatures" mean that it cannot be used before the Wild Beastmaster's effect is applied?
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Yes, you can make these work well together.
The ruling on Wild Beastmaster actually answers your question:
So you can have this sequence of events:
I think the relevant comp rule is 107.3c (emphasis mine):
Wild Beastmaster's ability does define the value of X to be its power, so that's the value of X the entire time it's on the stack, and it can change while it's on the stack - for example, when Trumpet Blast changes its power.