I can't speak to the 9800 as I don't have one but on my 2200 the print driver won't allow me to print on matte paper if I have a photo black cart in the printer.
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Same here. With both the 2400 and older 7600, the Epson driver won't let you print on matte paper with photo black ink; the matte paper choices will be grey'd out if you have photo black ink loaded.
As I understand it, the paper choice sets all sorts of parameters "under the hood" such as ink limits, grey channel replacement, and where in the curve cyan is replaced by light cyan, etc. The matte black ink in theory should give a better D-max on matte paper because its large particles of pigment sit on top of the paper, while photo black ink soaks deep into the paper and gets lost.
Interestingly, HP spells out explicitly what the paper settings are doing with the Z3100 printer, including which black ink is used, total ink limits, head height etc. in one of their technical PDF's. With this printer you can use both matte and photo black at the same time with some papers, and the effect can be kind of interesting due to the different reflectance of the two inks.