I just tried to do the same (generating a "scene-referred" image) with Adobe Camera Raw and the raw NEF image turned out reasonably OK (the white patch had RGB values of 190,190,190). Your TIFF file definitely looks off, way too dark. The interesting question then is, does the ACR generated file have gamma applied? Not sure... If you'd like, I could send you the TIFF file I generated. You could then perform the exact same generation steps and compare the results. Correct me if I'm wrong, but the absence of gamma application shouldn't result in a extremely dark image, but rather in a darkish, "flat" looking image. To be honest, my experience with argyllcms to create input profiles is limited to scanners. My camera profiles have all been done with the DNG profile editor, which requires RAW files (in the form of DNG) as input.