I still can't explain what's happening in PhotoPaint, but it makes sense to work at the highest quality level until you have to export to the web or whatever. If your output is not to the web, don't reduce the quality. 24 bit RGB should be sufficient and is the maximum that X can work at, as well as being the maximum quality of most printers. 48 bit RGB would only be an advantage if PhotoPaint can apply 48 bit effects. If the 48 bit format is just an export option then it's of no benefit.

As I mentioned above, CMYK is an export format. Unless your colours are defined in terms of printing inks or your next step is to send your artwork to a professional printer, you should avoid working in CMYK mode.

Regards - Sean