It always blows my mind how few 'bent riders advocate DF's for bad backs. I own two 'bents, and while I love them both, I don't consider either to be easy on my back (I have a couple fused vertebrae and scoliosis has been a PITA for as long as I can remember).

If I want my back to feel its best, give me traditional racing geometry. Between the different muscle sets that get worked and being stretched out, I'm way more comfortable on a race bike. So as others have already suggested, play around with different setups and see what works. You may well find that bikes that are supposedly designed for comfort are anything but, and that an aggressive setup feels great.