The contrasts look good, although I think a bit too dark now. Black paint on canvas isn't entirely black, and I've added some texture that now got lost in your dark areas. Tell me how it looks when printed!
By the way, I assume that most poster-services print in RGB (because they print digitally). PosterXXL says so in the FAQ (at least on the German page):
Which color-space should my images be in?
Please send your images in RGB- or sRGB-color-space for best results. All other color-spaces will be converted to RGB. This may lead to color discrepancies. Complaints because of these discrepancies are not possible.
(translation by me