QTR-Quadtone RIP

Hello.

About a month ago, I tried out Neutral K7 inks in cartridges on my R1800 using 
Hahnemuhle PhotoRag paper, and was very pleased with the results. I therefore ordered 
and have just installed a Niagara IV system in order to print up an exhibition.

To my horror, I find that for some reason, all my test prints are coming out inverted, i.e. in 
negative - whites as blacks, and vice-versa. Details:

1. The correct inks are indeed in the correct place.
2. I am using the Hahnemuhle curves for PhotoRag paper which came with Quad Tone RIP, 
and in general following all the instructions provided by Piezography to the letter.
3. The images are Greyscale 2.2 tiffs produced in Photoshop.
4. Some of the very files now printing in negative printed OK a month ago.
5. As a test, I printed the images through QTR, but without specifying any curves at all in 
the QTR menu. They came out looking muddy, but at least they were not inverted.
6. So the problem seems to be associated with the same curves that worked fine on this 
computer & printer a month ago...

I don't think I have done anything blatantly stupid, or forgotten anything obvious. As I am 
going quietly crazy, any help at all would be greatly appreciated.

Thanks in advance...