Hi numbnut:
Size has not been a problem.
My shelf is not 18 inches, but the feet (being inset a bit) allows the 990 to sit just fine (also the 755). I have a cabinet and took off the back years ago for access to wiring, physical connectors, etc. Also I added some "cabling-holders" [trying to think of a better name] in order to "bundle" my audio lines separately from the AC for obivous reasons. I even have a SONY DVD & CD 200 disc changer that's deeper than a 990 and it sits OK. So I wouldn't worry.

I had a solution in mind if the 990 was really going to be too deep [since I had made up my mind to get one anyway and work around it]. The work-around was to take a piece of finished 3/4" plywood and cut it to fit with the back end sticking out [if needed]...then screw it down to the existing shelf. Didn't have to.