The Yok spring set has a very narrow range from their hardest to softest. The front range is soft relative to other manufacturers and the rear range firm.
From what I remember even the blue rears (softest) were harder than AE silver (& i think AE grey). Made the world of difference with the BX when I had the springs measured and realised this. (after the season had finished and just before i changed car though
)
Found the BX transformed on blue rears with green or black fronts.