I see what you're doing.
Try these test swatches and gradients I came up with that noticeably shift when converting directly from ProPhotoRGB to sRGB.
Create a new document in ProPhotoRGB in Photoshop and make these RGB color swatches:
Navy Blue: R35,G10,B120
Cobalt Blue: R30,G20,B235...
From these create a white to color gradient for each and afterward convert normally to sRGB the entire document and see if you get a shift. If you do see a shift, try it using your JcSPACE or jc1RGB method described in this thread in order to reduce shifting of these colors.