ULA Circuit fits the bill.
I used an Atmos 65 on the PCT despite having an otherwise light load. Its a superb pack and especially nice in the Sierras with a bear can, ice ace, microspikes, and 11 days of food. Really good for the desert too with water loads and a breezy back. By removing the lid, trimming parts of it up, and replacing some of the heavy webbing with cord, I bet you could get it to 3 lbs. There are several threads on BPL about lightening packs for inspiration if you want to void the warranty and keep it.
I considered the Exos 58, but the padding seemed much less durable and substantial despite the desirable fabric weight.
To get into a smaller pack consider packing differently. I thought I had a pretty decent system of packing after 2 thru hikes, but it always seemed more bulky than it should. This video by Mike Clelland! packing for a trip with a tiny Golite Peak helped a lot:
It's aimed at frameless packs but I found the volume reduction and overall efficiency gains were significant. Try his style with your gear in the Atmos to see if it helps you too.
Heck, with the Golite.com sale going on right now you could keep the Atmos for heavy trips and pick up a Men's Pinnacle or Women's Jam (size large if you are normally a medium) for next to nothing.