Sorry, Greg, but it has to be more pull-ups. Think of the muscles you are trying to work... it's the Latisimus Dorsi, or "Lats." You COULD work at seated lat pull-downs, but the best thing in the world is MORE pull-ups. If you can only do 8, that's good... maybe next week you'll do 10, and that's progress. According to Arnold (and he IS right...) you need to do at least 50 pull-ups (total) for the exercise to be sucessful. That's a total of 50 reps for the whole workout.
One weakness that I have noticed among people trying to blast back muscles is the grip... your grip will typically give out before your back muscles do. To compensate, try getting ahold of a pair of those wrist straps and wrap them tightly around the bar before you start.
BELIEVE me when I tell you that if you focus on this you WILL improve, and people will start coming to you for advice on this when they see your results. When I was into this real heavy several years back, I would do two sets of 40 reps (YES!), a set of maybe 35, and then as many as I could do, perhaps 20 or so. This was my warm-up on the nights that I worked my back. After that, I would continue with lat pull-downs and move on to the weights.
Try to remember, also, to FOCUS on feeling the Lats contract, and squeeze at the top of the motion. It will do you no good to "cheat" using arm strength. You need to have the guts to go PAST the pain; you WILL see results from this, and it will further boost your ego. HTH...
FITTER out