I ride hundreds of miles with two bikes-- my Surly Long Haul Trucker (LHT) and my Cannondale Synapse.

With my LHT, I can ride for a long time with no back pain. The handlebars are very high-- probably an inch above seat level.

With my Cannondale, I get back pain after about 15 miles of riding. I've tried everything-- I got a 30-minute fit at the LBS store. I rode 500+ miles and still had pain. I switched to a shorter stem which I think helps, but pain still there.

At this point, I'm tempted to pop the handlebars on my Cannondale up higher to be more similar to my Surly. But this obviously slows me down and I want to ride fast on my Cannondale.