While I agree this is one notch below a standard Paper Plane, it is still a very good cocktail. Grapefruit obviously works really well with both bourbon and Montenegro. I might have to experiment with acid-adjusted grapefruit juice instead of the lemon, to see what boosting both the grapefruit flavor and the bitterness will do.

Replacing the Aperol with a grapefruit liqueur is honestly quite genius. I never would've thought of that. I honestly think, though, that you could make like a 2/3rd split of pamplemousse liqueur and 1/3rd Aperol just to get that bitter, bubblegum twang. It's still very good as is, it's just not as interesting as the Paper Plane to me.

Substituted 5 ml Amaretto with campari to 3/4 ounce for amaro which balanced well.

Sounds lovely. We make a Paper Plane with same ingredients but sub Aperol for the grapefruit. Great party drink in a pitcher served over ice!