Granted your level of comfort is up to you, are you a cold sleeper? Hot sleeper? That is a big factor in determining what colder weather gear to bring. It really seems like the underquilt is the way to go in the long run, however I don't have one yet and I have been fine to below freezing with a 3/4 selfinflatable pad, 30 degree synthetic bag, and good thermal base clothing with no underquilt or tarp. At least you don't have to wrestle with you pad with the Big Agnes bag, good luck.
Thanks for the reply. I tend to sleep hot. My bag is a zero, and the pad is a 15, and I have a liner. In theory, that should be more than warm enough for Oct-Nov. I'm kinda new at this, so I wonder a lot. Time and experience will tell.
The integration of the pad and bag are the main reasons why I bought them.