I think I found my sweet spot today (after 1300 miles). My butt was just a little too forward on the seat. Now, I make sure I am all the way back in the seat. This also allowed me to move the seat up one notch and tilt the seat back a little, which I prefer. The seat back now supports all the way from the bottom of my shoulder blades down, comfortably.

I now understand the "wallet" comments above.

 

it must be the way that you are sitting in the seat that causes the back pain.. have you tried adjusting the seat to a different way to see if it helps?
also search around.. someone added padding to the seat back ( an old pillow) and it apparently helped him a lot.. and the seat still looks 100% stock.. no buldges of high points

 

I'm 5'10 also and when I first got the S I found the seats very uncomfortable. I find out after a couple of months is that I had the seat tilted too far back. Try sitting more upright and closer to the steering wheel. For long trips move your wallet to the front pocket.

 

The angle of the seat bottom might be too flat. I had the same back pain and put longer bolts through the front mounting points, passing them through some large nuts that are about 1/2' thick. This subtle alteration made all the difference. I could only go about 1 hour before, 10 after with no pain. I know it sounds corny, but it also got rid of the lumbar pain totally. It is a very cheap fix, reversible, and easy to try.

 

To expand a little on what's been mentioned. You may have tried experimenting already but if not by all means try eliminating as much tilt in the seat back as you can. The seats are designed to fit you like a suit, so you want to get as comfortable as you can. Since I have some arthritis problems I raised my seat by 3/4" using grade 8 washers and longer bolts. That helped me a lot.

 

I had horrible back aches after rides in my mother's coworkers 02 and my friend's 03, but in VTECMOM's 01 or my friend's 01 I had no problems. I do not know why.
I am 5'6" 120