In regard to your questions:

You could calibrate at 85db instead of 75db. Your settings may end up at about +2db for the speakers and -5db for you sub, closer the center of the adjustment range. This causes no harm. You would just realize that the ‘reference level’ playback on your 950 would happen at a master volume setting of -10db instead of 0db. This is no great loss since various sources provide playback signal levels that are not entirely consistent from source to source anyway, making the pursuit of ‘reference level’ playback a never-ending task. I don’t bother with reference level playback myself. After all, the ‘correct’ overall level is what sounds best for the source and circumstances at the time you are listening.

Regarding stacking, by ‘on top of,’ do you mean directly on top or with two or more inches in between and air intake room as well from below the amp? If I were stacking, I would put the amp above the 950 with at least two inches freely open in between and nothing above the amp for at least six inches. A warm solid-state amp is nothing to be concerned about. If it is regularly getting almost too hot to touch anywhere on the upper vent area of the amp, it would be best to consider a different arrangement for better ventilation.

And yes, the ample availability of more volume without noticeable distortion is something that still puts a smile on my face. I didn’t know what I was missing until I went Outlaw! Enjoy!