Dmurfet,
FYI: My buddy used to have his M2.1s in cabinets as you have proposed.
The cabinet got very, very warm, but he said he never had any problems with them overheating, so assuming you have really good ventilation, I think it would be okay.
However, the idea of adding a silent fan makes sense to me.
(Personally, I have mine on amp stands next to my speakers, so I can shorten the speaker cable lengths.)
FYI: My buddy used to have his M2.1s in cabinets as you have proposed.
The cabinet got very, very warm, but he said he never had any problems with them overheating, so assuming you have really good ventilation, I think it would be okay.
However, the idea of adding a silent fan makes sense to me.
(Personally, I have mine on amp stands next to my speakers, so I can shorten the speaker cable lengths.)