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One day on a whim, I raised the back of the seat. My back was a LOT happier. It was maybe only a 5° angle and I never tried it the other way because if my back isn't complaining I'm a happy camper.

I find the same thing. On most bench seats, the front of the seat is putting pressure on your hamstrings.
Much of lower back pain is because of or along with tight hamstrings. Sitting in a canoe for hours doesn't help keep them (hamstrings) loose.

That little angle makes a world of difference. If you have a power adjustable seat in your car and find that on long drives your lower back or sciatic nerve is giving you fits, tilt the seat forward just enough so when your feet are flat on floor, there is no contact with front edge of seat and adjust the lumbar support if you have one to support the natural curve of your lower spine.If you don't have that feature, they make driving pillows just for this purpose.

It might make all the difference between arriving with your back feeling good or tight and sore.