Are those units deep enough that the tweeters will tucked inside as well? I wouldn't recommend this mounting idea if they are.

I would mount above the top of the tower by extending the back of the shelf higher up or box the top "shelf" area in rather than cut holes in the top plate and mount the QS to the front face of the now boxed space.
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Jason
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TTTHHHPPPPPTTTT!

My HT