I just got my new symmetrical IX underquilt from Mac last week and took it out for an overnighter up on the Mogollon Rim at Knoll Lake. I used a pair of mini biners to attach the shockcord at each end to the amsteel where it meets the whipping. I initially wrapped one end of the shockcord a few times to increase the tension because it seemed to conform to the bottom of my Traveler better, but later that evening I found that I got better warmth to just clip the full length shockcord without any turns to shorten it, presumably to increase the size of the trapped air pocket.
Here's a picture Te-wa took of me lounging in my Traveler that afternoon with the IX underquilt attached. Nothing like a little afternoon siesta!
When I initially went to bed the temps were in the mid to low 40's. I was amazed at how toasty warm I felt with a full length bottom insulation weighing only 8oz. The temps did plunge a bit lower than the weather forecast, and we estimate it got down to around freezing. At around 1:30AM I finally got too cold and used a backup down underquilt I brought along to get warm again. So my initial guess at the comfortable temp range for this underquilt would be down to around 40 degrees, possibly a bit lower. I'm really excited to have such a lightweight and comfortable solution for summer hanging. The quilt packs up very small too, around the size of a large grapefruit in a stuff sack.
Thanks Mac for pushing innovation and coming up with this exciting new product. This will shave another 5-6oz off my pack weight and decrease my pack volume by a lot. I can't wait to get out this summer and do some SUL hanging in comfort.