Hi Dave,
I ride a romanceur with drops, an appaloosa with drops and an atlantis with flat bars. I have not ridden a Sam.
Romanceur works fine on singletrack with both 650b with 2.1 tires (before) and 700c with 38mm tires (currently).
Appaloosa is on 48mm knobbies. It does feel a little more smooth and stable on the same singletrack, but the romanceur is still fun to ride. I guess I feel a little less "confident" on the romanceur but not to the point where I don't ride those trails.
The atlantis (2005 or so) has 38mm knobbies on front and 38mm file tread on back with 800mm mtb handlebars. Does the same single track and feels great.
The sub 40mm tires bury in the sand sometimes.
But all three bikes can do smooth single track and pick through rocks. Caveat: I mostly ride in ft ord which is smooth packed sand with very few roots or rock gardens.
Point is, any of these bikes work fine and you would get used to any of them. Maybe just think about what riding you're doing most of the ride and most of the time.
And a mtb would work better on single track than any of these, if you're really keen on trails!!!
- gabe
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