The negatives will help you increase your lat strength. Your eccentric (negative) is approximately 30% greater than Concentric (positive). In other words for example if you can curl 100 lbs you should be able to lower 130lbs with control. My point is...start adding weight to your negative pullups. You should be able to control an extra 25 lbs or so. This additional load will stress your lats a step further. I'll bet that within 2 weeks you'll be doing pullups unassisted.