This combination of flavors works really well together and is quite drinkable with more sensible proportions like 2:1:1 (2 parts rye), which is still pretty sweet. It also benefits from a couple dashes of black walnut bitters, as suggested by others, and from service over ice in an old-fashioned glass to keep it chilled for sipping and to enhance gradual dilution. I realize this makes it something different than what McDonnell may have been envisioning, but I just don't like mixing drinks beyond a certain level of sweetness.
I tried Amaro del capo, Borsaw Rye and Kopcke reserve port in equal measures, as this rye and port are not so strong and sweet. The resulting cocktail was very satisfying.
I challenge you to make this with any fortified wine, similarly to how you can change the amaro used. It may come out sweeter, the walnut bitters others have mentioned would balance it out nicely.
All great cocktails, in their simplicity and purity, irresistibly invite experimentation. With this one, we look forward to playing with the Amari and some bitters. To date, Nardini shines brightest as the Amaro for us.