All the answers so far are right on point. I've used a down bag in the eastern US forests for about 15 years, and never gotten it wet - so I tend to agree with oldranger; it's not that hard to keep them dry.

I've used an MSR Carbon Reflex 1 double wall tent for about two years now, and never had any significant condensation. I would get condensation in my Hubba, but never a lot, and the two wall design helped keep it away from the bag. (If your shelter consistently generates enough condensation that it drips onto your bag enough to get it really wet, you may want to consider replacing it - and that's whether or not you get a down bag.)

When I still used a tarp, my down bag lived inside my bivy sack, and stayed there inside my pack; I've never stored the bag in a stuff sack. I've always used a rain cover, and never had issues with water getting into my pack. (I even took a dunking in a creek crossing once, and nothing got wet except me.)

I always chose the tarp and bivy combination because I never trusted either one alone in wet conditions. Condensation was rarely a problem under a tarp pitched as an A-frame or lean-to (a weathertight pitch, or a shaped tarp like the Silshelter, were prone to heavy condensation.) With the tarp alone, there was too much blowing mist and rain to be sure of staying dry. With the bivy alone, there was no way to enter or exit without exposing the bag to the driving rain. Using the combination meant that I was always sure my sleeping bag would stay dry, and I used this combination for years, until the Hubba came out and was lighter and more efficient (for me) than the combo.

My experience is heavilyl skewed toward the eastern US. I hike and camp in Ohio, Indiana, Kentucky, West Virginia, and western Virginia, with occasional trips to Isle Royale. I've never taken a trip into the Smokies or your area, so you need adjust my advice for that.