If nobody else noticed the problem, maybe the problem is with something you are doing and not the machine or the operating system.
I would like this to be the case, but I'm afraid it's not. Or at least it's not so simple. First a little background: I had a 7800 and a 9800, then about two months ago I sold the 9800 to a friend of mine because I need a larger printer (the 11880). A client of mine sent me a very high quality scan of a painter artwork (true 600 dpi, 800 MB TIF) to be printed on canvas. While making test prints I noticed that sometime the printout was softer. It took me many hours (and much ink and paper) to pinpoint the cause to the use of a default paper size vs. a custom one. Then I took Bill Atkinson's test image and used it as a reference, printing on many different papers, including Epson PGPP250. Same result. I downloaded and installed the Windows driver under the copy of Windows XP wich I run under Parallels on my iMac, and this time the results are the ones expected, i.e. no difference whatsoever between default and custom paper size. Note that I used exactly the same image with exactly the same driver settings.
So I went to my friend's studio to repeat the tests with my old 9800, and the results were absolutely consistent with mine (no difference in Windows, softer on custom paper size when printing from OSX).
After that I went to a certain number of print shops with an Epson x800 machine and OSX, and asked them to print my target image two times, one on a default paper size and the other on a custom one. Again, same result, softer on custom. Note that at this point the test had been made on various combinations of OSX (10.4.9,10.4.10,10.5.0,10.5.1), processor types (PowerPC and Intel) and Epson drivers.
I'm in Europe, but just to be sure I downloaded firmware and drivers from the Epson USA site and tested them with my own 7800, but the results were, again, the same.
Meanwhile, I've contacted Epson, and their response was vague, with something like "maybe you are making mistakes in Photoshop", "maybe you have clogged nozzles", "print from Windows or buy a RIP".
Yesterday I made another discovery: the degree of softness depends on the size of the margins you choose when creating a custom paper size. If you click on "
Manage custom sizes...", then "+", you end up with an Untitled size with margins fields already filled with 0,63cm for top, left and right, and 1,45cm for bottom. If you use these values, changing only the size of the paper, the printout comes out sharper. Not as sharp as a print made with a default paper size, but in the middle between default paper size and custom paper size with any other values for margins (Photoshop CS3 10.0.1, OSX 10.5.1, Epson driver 6.11 downloaded from UK site).
At this point I don't know what else I can do.