Every time I feel an inkling of back pain, I start doing exercises that strengthen my back muscles. If you are already at the point that pain is chronic, then you need to see a medical professional and I very much recommend using a physical therapist too.

If you can try a bike before you buy, then you should try all that appeal to you no matter what they are marketed as. But keep in mind that the most comfy for short rides may not be the most comfy for long rides.