Thanks, all. I hadn't considered that the rims could be too light. My use will be some extremely mild single track and gravel roads, but you've convinced me to reassess the tubs for that use. The 730 is already slated for a 6600sl build and I've got an ultegra wheelset ready to go on it. I may end up selling the tub wheelset and getting a nice sturdy pair of clinchers for the CX build.