... fit into 17 X 8 X 19 inch space in Stereo closet and run cool, budget under $2K used.
Is it possible to take the $2k that you're going to spend on a amp (that will most certainly not compete with your wonderful Atma-Spheres) and instead spend it on modifications to the stereo closet that would help the amp run cooler? I ask because...
I recently moved my equipment into a dedicated closet just outside my listening room. My amp is class A, so I was worried about heat. My solution was to place the amp on the top shelf of a tall rack and have the closet ventilated from above with a large but quiet cooling fan. The fan sends the hot air through a duct in the attic to the exterior of the house. A one inch space between the bottom of the door and the hardwood floor acts as an air intake. Even with the door closed all day and the equipment on, the closet doesn't get warm. Total cost of construction work (not including rack): $300.