I can reccomended this , but you do not have to listen to what I am saying. I do stack them but with about 4 inches of space between them and a smal boxer fan on the lower half unit witch ever one that is. In this way nothing gets hot just warm. When I did it as kingwa said , it was hot enough for me to smell the electronics cooking. To hot for me to use this way. Now this is just my opinion. And yes pick one volume control only all others should be turned up to max. Example would be to use the volume control of M9 only. Volume controls tend to rob you of audio performance .
Hi - thanks for your help. I don't have the space for any fans :( I guess it's okay in the winter, but summer time is going to a problem. Funny because the M-7 doesn't get anywhere as hot as the M-9.
I'll put volume full on my stereo amp and control from m-9 then.