Hi BrendanJ,
I think correct form is the key here.
Seated cable rows should be done with a straight back-chest out-stomach in and a slight bend in the knees to take the load off your lower back. When pulling the 'V' bar or handles in make sure you pull them low and to your waist, pause for a second and let the V bar or handles out making sure you fully stretch your lats forward. Do these nice and slow concentrating always on you lower lats, always keeping the V bar or handles low and drawing you elbows in tight to the side of your body. Flaring your elbows/arms out will also throw the exercise onto your arms. Using a straight bar will put more emphasis on your arms. Weighted back extensions will not work the same muscles (Lat's), they are for your Erector Spinae and not your Lat's.
When doing pull-downs cocentrate on drawing your elbows down and in as you pull the bar down to your chin or behind head. As with any exercise always focus on the muscles you are working (upper lats) and do them slow especially on the negative ie. letting the bar back up. Any exercise will always hit the weakest spot first, but if you do them correctly you've got more chance of fatigueing the right muscles.
Good luck!
