Without seeing the space, I can't offer much in the way of alternatives. Maybe an amp stand, perhaps next to the sub?

As for placement on top of the sub, I can think of only two things -- I assume that the top is a flat surface with no ports or cone in it, and I'd recommend checking your binding posts and cables from time to time to make sure vibrations don't loosen them. One thing you could do is include some vibration isolation -- neoprene pads under the 7100's feet could offer some decent dampening. You could probably get some 1/2" or 3/4" thick neoprene at the hardware store and create some isolators pretty inexpensively. That would reduce the likelihood of your cabling getting shaken loose and reduce the vibrations rattling around the cables and circuit boards inside the amp itself.

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