Thanks for the detailed information Andreas, and I apologize for not responding sooner (somehow your last message got lost in my email).
We are in the process of deciding on a new Piezography sample image, and may make one similar to Keith's with a few different images, gradients, etc...
We are also in the process of testing this recent 3800/3880 dark output issue, as we've received reports from a few customers. So far, we have noticed Mac 10.9 prints MUCH darker than previous operating systems, and we are continuing testing with Mac and Windows platforms to trouble shoot and resolve this issue.
Based on the information you provided, your settings are all correct, and I would be happy to make you a free custom curve that is specific to your exact setup, which I believe/assume will give you accurate/linear output.
Please go here to download the custom curve package, including the printable target, master curves and instructions. For glossy paper, please use the x800-x880-GLOSS-Master curve, or matte/rag papers, use the x800-x880-MATTE-Master curve.
Please let me know if you have questions or there's anything else I can help you with.
I will let you know when we have more information from our testing, and assume we'll send out a newsletter to all our customers at the same time.