If you can find them, the older Marantz MA700, MA500 and MA6100 monos can typically be found pretty cheap and they are solid amps, if you don't mind used. They aren't shorter, but three will take up even less physical space than three 2200s (except the MA700s). I ran three 500s for years.
Since it is an open rack, 9" is probably adequate for the XPA-3 as long as there's a few inches on either side as well. Here's how I have mine:
My side panels are perforated metal, so they let air flow and as you can see, the back is removed also because my receiver and amp are too deep for the standard back plates.