I was just thinking. This new idea may or may not work for you, but with your small space restrictions, you could look into the newer Class D ncore amps, such as ATI Technologies AT542NC. It’s a two channel amp with fully balanced differential (bridged) amp modules. I’m not particularly a fan of Class D amps, but these seem to be better than the others and you do have severe space restrictions. The AT542NC would fit perfectly because it’s only 10-5/8" deep.
The Nord ncore amplifiers use a discrete Class A input stage (using either Sparkos or Sonic Imagery discrete op amps). These would be good for your situation. Their monoblocks are 9" deep, stereo models are 13" deep. Being Class D, you should be able to stack them no problem.