QuoteGenerally a "racing style" bike - meaning one with drop handlebars - is easier on the back, since when you hit a bump the spine is curved and more able to absorb the shock. Depending on the sort of curvature you're talking about, that may be part of the solution.
The unexpected truth. But actually I wouldn't say "curved" particularly because I ride in a very low position with a nearly horizontal back on my aerobars all the time, for hours on end, but my back is still relatively straight; but the fact is that the back has the vertical flexibility to take the bumps in that position without damage, whereas sitting up straight is just asking for trouble with bumps putting extreme compression on discs since there's no give in that direction.
I agree, but also advise caution in the beginning if necessary. Start out as low as comfortable, keep upper body relaxed, elbows bent, shoulders not hunched, and fingers draped loosely over the bars. Ease the position lower as you adjust to riding more mileage. Add that mileage gradually and change position a little at a time. You need to decide how low is low enough, but riding very upright for long distances is hard on the butt and hard on the back.