my back used to get sore as well and i found out it was cause of my form when doing squats.I was getting the bar off the rack incorrectly and i wasnt breathing in deeply before going down.i changed those two things and now no pain.
i also chaged from stiff legged deadlift to the version where you start with a slight bend in the knee and straighten it as you pull the weight up while keeping the bar slightly rubbing against your shin as you perform the exercise.
proper form is key.ive been lifting since 98 and i thought i knew what i was doing all this time but the more i read the more i realize i had been doing some things incorrectly,so my advice to you is to read and watch videos.
Hey bro. On the SL 5x5 you do your warm up sets. Then you put on your working weight and do 5 sets at that weight. Do not change it.
Like this.
WU bench 2x5@135, 2x5@185. Work sets 5x5@225 Just an example.