The correct technique for seated row is pretty simple. http://www.getfitchimp.com/exercise/seated-cable-row
- Keep your knees slightly bent to reduce stress on your knee joints
- Do not hump your upper back as this will increase stress on your shoulder joints
- Do not hyperextend your lower back when you finish as this may put more stress on your lower back
- Pull close to the abdomen has been shown to reduce stress on the neck
Regarding different types of handles, EMG studies show no significant difference regarding muscle activation (lats, rhomboids, upper traps and biceps). However, closer to the body with underhand-gripped has shown to activate the biceps, rhomboids and middle traps a little bit more. Do not allow your elbows to flare outward as this will increase the stress both on the shoulder and the elbow joints.
You might want to add bent-over rows (barbells or dumbbells) since these are the most effective exercises for the lats and upper back according to EMG studies. If you haven't yet, you should try some prone T's to isolate your middle traps even more. You can google or youtube this exercise. If this is too easy, you can try to perform the prone T's on a stability ball. Hope this helps!