Quick little story. Riding 58-60 cm frames, down tube shifters are doable, quick, efficient, but never liked one armed riding on gravel. Always preferred bar ends especially on gravel and especially touring. Balanced, lowers center of gravity and puts a hand on the bars which keeps you traveling straight and secure. Tried Riv's stem shifters a long time ago but the raising of the torso to get leverage and one handed riding was uncomfortably iffy. Maybe I didn't give'em a chance. Then, a few years back I tried top tube shifters on a vintage 50's British tourer (either a Gillott or Ephgreave, or Taylor or one of those gems designed and built in Heaven). These were not right behind the head set. They were set back maybe a third of the distance on the top tube so I didn't have to reach forward but rather back, to a point just forward of where the knees come up. Didn't have to lift the torso. Plenty of leverage. In fact, it was like my arm became a part of my torso and added to the stability of the bike. Overall an interesting and pleasurable riding experience. And there you have it.
Craig in Tucson