I have owned a K75S and now ride a K100RS. Neither bike had bar backs even though I have lower back and neck damage from accidents. I find the K bike riding position fairly comfortable for up to 4-5 hour rides.
If you are experiencing back pain when riding, you may be better served by using some sort of back support. In the old days that was a "kidney" belt, but nowadays you can go to the drugstore and get a back brace that does pretty much the same thing. My riding pants have built in back support that keeps me comfortable even for Iron Butt rides.