I am not sure I understand everything you say. But I agree: HW are very, very neutral. I changed the cartridge too (X2) and there is a slight improvement in stiffness... 3 pin cables can be stiffer, actually; you are again right... But for the Voile red cartridge, they are the same as the blue or green ones; they are used for exemple on the Voile straight HW (without the pins). Or do you mean fitting the Rotte red cartridge on a HW? What you could do is use your Rotte cartridge and wires in a 3 pin cable binding. It works!
Well, the red SB cartridges are stiffer than the blue HW ones, I've compared personally. Maybe the old reds were the same but this doesn't seem to be the case with a the HW and SB now. Also the a Rott will fit on the Voile 3pin toe piece but not the blue HW, the rods hooks won't fit in the HW pivot attachment point thingies.
Ok, so someone educate me. I always thought activity was a function of location of the pivot point. The 3pin and HW share the same location for the pivot point, the SB are forward of this point and therefor more neutral, the X2s are further back and the most active. For me the 3pin/HW seems to be the sweet spot. I never liked the SB, and the X2 was great, but a tad much.
So now I'm fiddling with HW and black cartridges, wondering what people think, and when I'm worried that they are too stiff because I feel like the boot is compressing too easily and am I justified or just not used to it?