Hi Dana and mrmoose,
Based on my experience with my 7900, when a contiguous section (with multiple complete lines) of a channel suddenly drops, and repeated cleanings result in no changes (or result in further expansion of the block of missing nozzles), you have a failed head. It's not really a clog; the problem is that the head is no longer working from an electrical/mechanical (i.e. piezo) perspective. You can read about my experience hereon this forum.
I chose to repair my printer myself, since I had the necessary background in working on equipment like this and I had the required tools and service documents. I replaced both the Ink Selector (which contains the 5 dampers) and the head. Like Dana, I found the www.myx900.com website to be extremely helpful, and of course this forum (Dana, Jon) was also helpful! The Ink Selector was sourced from Compass Micro and the new head was purchased directly from Epson.
Some might question the repair as a bad economic decision, but for me it was the right approach. I like the quality of what this printer can produce and I wasn't interested in changing horses. I also like to know how my tools work, and I feel like now I can tackle anything on this printer based on what I've learned in the process. My head failure occurred after 5 1/2 years of excellent service from the printer. With the repair, I am expecting to get at least another 5 years out of it. And because I spent time doing ALL the "post head replace" adjustments as recommended by the service manual, I have a machine that is now optimally tuned and printing like new (maybe even better than new). If you follow this path, I recommend you re-profile the papers you use after replacing the head, since a new head can actually cause a change in color characteristics of the printing system (similar to how a change in Media Type settings can affect the system output).
But I'm fully aware that for anyone running a business with the printer or without the means or time to do the repairs themselves, it is probably best to favor the economics part of the equation and purchase a new printer.