I've been using an Epson R1900, which I'm generally very pleased with, for several years. I use Lightroom with a well-calibrated color workflow and usually get excellent results.
This weekend I was at the Oregon Coast Aquarium in Newport and shot this image (Canon 5D3, 24-105L@50mm, 1/30, f/4, ISO 1600)
When I printed it on Epson paper (with the matching Epson profile), the nice blue background came out a sickly purple, even worse than the color shift indicated by Lightroom's soft-proofing mode. I happened to have some non-Epson paper (Ilford Galerie) available, so I tried that (with Ilford's matching printer profile), and the results were actually reasonable, somewhat subdued but still quite acceptable.
I would have expected that almost all of the print gamut was inherent in the ink/dye, with only a minor effect from the paper. My sample scan below shows that the paper can have a HUGE effect on gamut.
So, my question is this: What is happening at the microscopic / chemistry level that could cause this much shift?
Here's a scan of the two prints side by side. The one on the left is Ilford Galerie, and on the right is the Epson Premium Photo Paper-GLOSSY. This is pretty close to showing the absolute colors, at least on my calibrated monitor, and gives a sense of the difference between the two prints.