you do concentration curl negatives if so thats something new for this thread atleast. Most people do dumbbell or barbell curl negatives. Lets make sure were thinking of the same thing by negatives you mean your only focusing on the negative portion of the movement? If so thats advance stuff and good for you. but I'd say stop going to burn out, decrease the rep speed, and really focus on the contraction throughout the movement even during the negative.
and to stop the pain in your lower back while doing standing curls is easy. CHEAT swing your hips it takes the stress away from the lower back
ps. i don't train biceps anymore