A lot of back problems are attributed to the footstool but are caused by something else. In fact the first back pain cause for guitarists is the angle with which they hold their guitar. For most, the plan of the guitar is almost parrallel to the plan of the lower part of the body, so they have to twist their spine to the left in order to play. It is hard to explain in words (and harder for me, I speak french) but look at it the next time you play. That twist of the spine is the cause of the major part of classical guitarists's back pain. The solution: hold the guitar otherwise. For me a simple change in the angle I mentioned ended in the disapearance of my back pains. I had to adapt to a neck farther of my face, but in the end it is just perfect because the left arm has more efficiency in that position.
So just make sure your shoulders are in line with your hips