We've been considering putting another brace across the lower bout to maintain the radius on the back but we really like what the flexibilty of this back does to the coupling effect with the top. You can really feel the back vibrating as you play them so we're loath to add any more stifness to the backs.
The first pic is an SJ, the others OMs. Not much difference really as they are both 15" across the lower bout.
Bob



