black position=shade 1
cyan position=shade 2
lt cyan position =shade 3
magenta position =shade 4
lt magenta position =shade 5
lt black position =shade 6
yellow position =shade 7
This is standard K7 ink shade placement in all 7 channel printers. And it carries over into all 8 ink channel printers with LLK becoming a GO channel or not used and is filled with PiezoFlush.
There are other ink sets besides K7 that have different shade positions. But, K7 shade placement is always the same.
If your inks are not in these positions, they will posterize with K7 curves. If your inks are not fresh, or if they have been sitting in the printer for longer than 6 months without use, you will get posterization with K7 curves. If you have misfiled ink cartridges so that they are partially correct and partially incorrect, you will get posterization with K7 curves.
When the 7600 was in active development, the base curve was Hanhemuhle Photo Rag. So, it is the first choice for using. Enhanced Matte would be second choice. As Richard points out, using something approximate for something that is very exacting is not necessarily the best thing to do. We certainly did not make any curves for cheap off shelf papers.
However, because you mentioned that you are getting a passible print without selecting a curve, this indicates to me that you may have not installed the K7 PPD. Or if you did, that there may be some corruption in the install. If you are selecting the Enhanced Matte Curve it would only be called "K7-2880-EpEnhancedMatte.quad". What surprises me is that usually not selecting a curve results in an extremely-dark, over-inked print when using the correct Piezography workflow without a curve. If you are not using the correct K7 workflow, it is possible you are getting unintended results.
In any event, the best place for support of Piezography is at Inkjetmall in our technical support forum: http://www.inkjetmall.com/tech We have all kinds of forums there discussing not only Piezography but also hardware.
You will need your customer number to post questions, but you certainly can read it and download the various technical articles. I suggest you register and post and one of us will try and help you.
best regards,
Jon Cone
Piezography