Been riding/racing since the early eighties and have had plenty of back/neck issues with ill fitting bikes. Lesson learned...."Get a professional to fit you!!!"". I don't know the area you are in but most good shops can fit you properly. Back pain can come from the wrong seat angle/placement, incorrect cleat placement on your shoes, wrong bars, seatpost, stem, frame, and technique. You'll spend less money with a good fitting, that taking guesses on parts replacement. Your stem height/length, or taller bars might make an initial improvement, only to shift the problem somewhere else.
Example...my road bike NEVER causes me pain except from rough roads (it's a crit bike, but is custom built and fit). My latest off-road bike felt like it fit but for some reason caused my lower back to hurt after a couple hours. A trained tech had a look and a tweak, and figured out the problem.
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paul, texas KD5IVP