I used Damson plum jam bought from a local orchard. I agree that it was a bit more bitter than I like, but think it was more about balancing the lemon juice and simple syrup. The jam with apple brandy is fantastic.
I picked up a bottle by accident, and on other’s advice, stored it in the freezer. At that temperature it’s quite drinkable either neat or on the rocks. It was a great amaro to use in the Italian Chocolate Martini recipe on this site.