Stacking shouldn't be a problem but I would remove the top and (if possible) bottom covers to improve air flow. You could probably disconnect the tiny (and noisy) stock fans and put a bigger (and slower) mother under them -even try locating the fan elsewhere and channeling the air through some 5-inch dryer venting hose(s). Come to think of it, with the covers removed, they might not even need forced air cooling. Give Len' a call and see what he has to say...
Amplifier Orientation?
I've got a pair of ARC VT100 MkIIIs and some space issues. One thing I thought about was welding up a rack to hold them vertically, and stashing them behind my front end equipment, which is sitting on a couple Solidsteel racks. Its not like there is any reason to actually see them... I can't intuitively believe there would be an issue with operating an amp in a different orientation--it might even improve airflow around the amp. Just thought I'd see if anyone had any wisdom on this grand plan before I embark on this scheme...