so if I like the size of the Jam but want a frame, I should probably go with either the Circuit or the Ohm?"
Yeah one of those or the Gossamer Gear Gorilla. That pack is a bit smaller though according to the specs.
I went with the Ohm (22.6oz )because it's a lot lighter than the Circuit (36oz) and rarely will I be approaching the weight limit. The Ohm is rated at 30 lbs max versus 35 lbs for the Circuit. With the Ohms new hipbelt, I bet you could push it a bit past 30 lbs though. The Ohm also has the superior compression system which is nice if you're going to have small loads sometimes. The Circuit would probably be the ideal choice if your pack is usually going to wind up in the 30-35 lbs range.
It seems to me that the ULA packs may be larger than claimed, as opposed to the Jam being smaller than claimed. My previous pack (MLD Revelation) was rated at 2000ci + 400ci and my new Ohm is rated at 2100ci + 500ci. You wouldn't think there would be a large difference, but my Ohm is way bigger. I can now carry my 2 person double wall tent inside the main pack bag and still have as much or more room left over for food. With the Revelation I carried my tent externally.