The donk logic confuses me (Stuper hand)

[Reckless1der]

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I guess I'm a donk, because if I were the BB and had A9, 99, or 55, I'd be betting between $250-$300 into that pot.

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I don't think OP is calling the BB a donk, in fact what he says is it's so early that he doesn't have reads. Instead, I think he's using the word "donk" to describe the type of bet that was made (EP and ahead of the original aggressor).

Personally though I think the correct play here with a set of 9s or 5s would be to check raise. With 3 players yet to act, it would be highly unlikely it gets checked through, especially with the Ace on board. With a set, you let someone make one more play to collect a little value then close the door with a check raise. I think this is what is puzzling the OP, a set would either probe or check raise, so does BB have two pair? would he PSB a flush draw? Do you think I have 6 clean outs (2-As, 3-Js, 1-BDNFD).