pullups/chins work the entire back e.g gymnasts exeptionaly strong exceptional back development tons of pullup/chin movements
do not use a weight belt for any of your training your lower back muscles must strenghten along with everything else
if u carnt do 5 chins/pullups at once do 5singles rest pause they are far superior to rows especially as u say u wont strength perhaps alternate...
change the pulldowns to pullups/chins change the rows to deadlifts exelent exercise mix it up by doin sum explosive movements eg...
