I also have back problems. I used flat mtb bars with barends (on a commuterized mtb) for several years, only occasionally riding my old road bikes with drop bars, because they were too low for comfort. Eventually I came to appreciate touring frames for their practicality (rack & fender mounts, load capacity, neutral handling). Once I raised the handlebar well above the saddle I discovered the drop bar could be more comfortable than flat mtb bar. Larger frame size, longer HT length and the accepted notion of bar-above-saddle on tourers permitted me to experiment in this manner. My drop bar is 7cm above saddle now, and although it looks funny it works well. I find myself riding comfortably in the drops quite a bit, but mostly on hoods. The flats (hand position on drop bar) are a very upright position with this frame/stem/bar/brifter configuration.