I think cutting out the ceiling of your rack could work very well. You don't even need more shelves, cut out the ceiling, stick one of the amps at the top and let it vent into the open space (not sure what is above your rack in the wall)

This way you can keep everything as is, but you'll just have a hole at the top of your rack that no one will see. This will allow you to have 6" above the bottom mono because your top amp will be right against the ceiling of the rack venting into the open space?

just a thought.

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