How do I sweeten a mulled or spiced wine that is too tart? I prepared a spiced wine according to a recipe last night using Cabernet with a mix of Pinot Grigio and Rum. I suspect the Cabernet caused it to come out on the tart side after reading this question.

The recipe called for brown sugar, which I used, but it doesn't seem sweet enough. Should I add more brown sugar or is there a better way to sweeten the wine and temper the tartness?

Thanks in advance.
I've used both sugar and orange juice to sweeten mulled wine in the past, but obviously orange juice would change the flavour significantly. Add more sugar slowly, tasting as you go.