I too use a Big Agnes with a pad in the sleeve. I also keep a fleece blanket in it for any cold spots or just as some extra warmth. So far so good. I can't justify the cost of an underquilt (with the exception of a poncho liner under quilt) at this point in my life sooo....
"Mother Gue", I says "the Rocky Mountains is the marrow of the world," and by God, I was right. Keep your nose in the wind and your eye along the skyline.