I don't know how you can say these things. The steel alloys you have mentioned all have the same stiffness, but differing parameters of strength. The tubesets made of these materials all have the same outer diameters, comparing tube for tube and allowing for some very small differences. The tubes mainly differ in wall thickness and i some cases interior surface design, such as using helixes. The 753 alloy is just as stiff as the 531 alloy, but 753 is stronger. 753 tubes have thinner walls than 531 tubes, and hence are more flexy (less resistant to twist or bend) than are the thicker 531 tubes. With equivalent joining, tube sizing, and use of the butting, how could 753 make a stiffer main triangle than 531? It doesn't make sense.

More stiffness can be had (without going to oversize tubes) by using tubes with thicker walls, such as Ishiwata 022 versus 017, Columbus SP versus SL, and Reynolds 531 ST versus 531C or 531 Pro. The most obvious way to get a stiffer frame is to go to oversize tubing with short chainstays. Then the price point shifts to favor aluminum.