Thanks for the info, obie. I figured there had to be a limit, but I would have hoped it would be more like 3-4 units in a stack. If the issue were just ventilation, it should be possible to add some kind of spacers to leave more of a gap between units in a stack, but there's not much to be done about the weight.

The fact that the M-Block is only 1 rack unit high sounds great, but if you can't stack more than 2 of them, and they don't have brackets for rack mounting...