Vertical subwoofer placement is often ignored because it's impractical, but there is a school of thought that the ideal position is about two feet off the floor. It's no coincidence, this is normally about 1/4 of the way to the ceiling, which is ideal for combating the 50-60Hz 'room boom' associated with this dimension.
Or in other words, unless it's vibrating the crap out of any components in the cabinet no, it's not a problem.
Russell
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