I understand the question about racks... I don't use one for my bass gear but do use one for my acoustic rig. 
For that rig I have a Dtar Solstice preamp and a Sunrise stereo buffer mounted onto a shelf - I removed the feet from the Sunrise and used those holes to bolt onto the shelf and the mounting hole of the Solstice. The reason for the rack is that I also have a rack mount compressor and digital effects unit as well. It was easier to consolidate everything into a rich than to carry them separately or replace everything and put together a pedal board. Still, the rack is big and heavy and expensive. Sounds great though.
Bottom line is that individual situations may call for individual solutions.
For mounting the MG800, you could remove the bolts/screws in the feet, replace them with slightly longer versions and use those holes to connect to a shelf. Leave the feet in place and you'll still have the air flow underneath...

For that rig I have a Dtar Solstice preamp and a Sunrise stereo buffer mounted onto a shelf - I removed the feet from the Sunrise and used those holes to bolt onto the shelf and the mounting hole of the Solstice. The reason for the rack is that I also have a rack mount compressor and digital effects unit as well. It was easier to consolidate everything into a rich than to carry them separately or replace everything and put together a pedal board. Still, the rack is big and heavy and expensive. Sounds great though.
Bottom line is that individual situations may call for individual solutions.
For mounting the MG800, you could remove the bolts/screws in the feet, replace them with slightly longer versions and use those holes to connect to a shelf. Leave the feet in place and you'll still have the air flow underneath...
