What cartridges does the cx6600 use? are they common to other Epsons?
I am ask bacause, paper type is not so critical a to preclude the use of third part makes, no matter whathe printer manufacurers says. You could try for example Ilford Galeria - from click here
I wonder also if you have manipulated the paper choice correctly - you could try for exampe on Gloss paper[ any make] printing on the 'Transparancy setting' This uses less ink and gives generally brighter image.
The thing to do is to Experiment