Use either of the neutral-grip handles to put your arms in a stronger position. While you'll be able to use more weight, remember that your arms bear some of that extra weight. When you use the narrow grip, your chest also does some of the work. Keep in mind the extra muscle involvement, and don't make these neutral-grip movements the only back exercises you do.
Use a stirrup handle to do your pulldowns one hand at a time. The movement will be more of an arc.