I like this one very much. It was just a tiny bit too sweet for me, so I added 2 dashes of Angostura. Balances it out perfectly for me. I will try it with orange bitters next time though.
I didn't find it too sweet, but I tried as your suggestion first without then with 2 dashes of Angostura orange bitters. As I am a huge fan of the "original" Brooklyn with Picon Amer and its orange notes, the addition of the orange bitters ever so slightly elevated the drink for me.
I would rate this a 4 using your sweetness scale. Starts with the Bourbon flavor (Knob creek 9) but quickly switches to the sweetness of the Amaretto and Vermouth. Finishes slightly dry.