some of it might be related to form. I personally do not use squats as a 'full power' exercise, and more of a mobility, balance movement, so I don't push as much as a could, partly to avoid this back pains you mention. I use single leg press a lot though, and after reading the UP!UP!UP! book it sounds like that is how they (the author) uses it too, squats for overall core strength and the press is the bread and butter of leg power.