i have problem with fountain fill when i export image i see steps... I use RGB and CYMK but there is still problem. Also try JPG, PNG, BMP etc...
Please understand that no image format can have more than 256 shades. Are you viewing this on a LED or LCD monitor?
Import your image in CorelDraw, crop into a small slice such as the one below, and slowly drag the eyedropper down along it.
Maybe you could also set CorelDraw into view > pixels and zoom in very closely.
If you have an optimum blend, then the R, G and B values for each pixel should each differ by no more than 1 from its previous value. This is the most that a normal 24 bit RGB image can support.
You could also try imaging to 48 bit TIFF -- but even with that, you would need something better than a true colour screen to be able to see the differences.