I think if you look through the NAHBS tandems, that most of them are using a larger, round, 4130 tube. I love the way the old reynolds oval boom tubes looked, but they aren't as stiff. People are used to it now that tandem builders have been doing this for decades. You aren't bending the boom tube in the major dimension of the oval all that much. I think they did it that way so they would fit a larger tube on a bb shell, and that's an issue with a round tube of similar section. You have to remember that when the Reynolds tandem tubing set came out, it was the only set of tubing with oversize tubes on the market. We were so set in our ways that Klien actually sued Cannondale over using larger section tubes, it was so unheard-of. I find it hard to get over the old ways, but I usually manage to force myself if the new ways are better or even just easier to source.