Thank you all for your advice.
Although I have never felt any coolness from below while sleeping on the Exped synmat UL7, I guess I (wrongly) thought all was well w/my sleeping pad choice. Truth be told, I've made decisions about sleeping pads based solely on weight and general description (e.g. 3-season or 4, presence of insulating material, etc.). I now realize the significance of Rvalve and the fact that even though I wasn't aware of any coolness under me I have likely been losing heat through the base/pad of my sleeping system.
I agree, my quilt should be adequate in the high 20s-low 30s (oh, and I forgot to mention I used a silk liner too, which should add a couple of degrees!). Replacing the pad will be cheaper and lighter than replacing the bag/quilt. The NeoAir Xtherm is lighter than the Xped downmat, so I think I'll go with that (even though I've slept on a regular NeoAir pad a few times and disliked horizontal design) and see how it improves things.
Oh... I do sleep in a tent (Big Agnes seedhouse SL1) and when the fly is on, I stake it out/vent it. Also, I realize sweating in all my layers is entirely possible... but I only started layering up like that in the quilt because I was so cold early on in the trip (started out with only thin top/bottom long johns, socks, and hat). I was sleeping so poorly, I hiked off the trail at Red's Meadow and went into Mammoth to buy heavier long johns. Ugh. (Oh, and I nearly always take a quick sponge/splash bath to get the day's sweat off & avoid nighttime chills).
I'm feeling hopeful... Xtherm, here I come!