Slouching is not a good thing to do. It will give you the back problem you are talking about and also will fatigue your entire body. I can tell you this from my own personal experience as well as from observing my students.

I use an organ bench. It is higher than a piano bench. But it all depends how high your keyboard is.

Main thing is you either need to sit higher and/or lower your keyboard.

Once you experiment finding a better relationship between the keyboard and your bench, experiment moving the bench in different positions closer to the keyboard than normal and gradually move the bench away until you find a comfortable spot where you can sit straight. Once you've done that try to find your center of balance by tilting a little forward and then try tilting backward.

With a little experimenting doing this you will locate your center of balance and be able to play for hours.

I used this to correct one students problem who had a very bad back problem. She was older and had deteriorating vertebrae. She could only play for 15 minutes and had to stop. She was going to give up playing because the pain was unbearable.

I had her go through the above series of movements and we found her center of balance which allowed her to be comfortable sitting at the organ.

I talked to her a couple days later and she was so happy that she was now able to play a few hours at a time.

Hope this helps.

Best
Scott Langholff
http://ScottLMusic.com