I would have two concerns: vibration and magnetic field interaction. Put your hand on the sub and on the McCormack during a loud bass passage. If you feel significant vibration, that's not good. Most 'philes would agree vibration of an amplifier is undesireabler. For magnetic field, see if the sub is magnetically shielded. If not, the magnetic field from a large speaker magnet could affect the McCormack's signal and eventually magnetize its iron-based components.
One way to have your cake and eat it too would be to put a large space between the two components -- raise the McCormack. For example, you could straddle the sub with a small, two shelf bookshelf or a rack and put the McCormack on top of that. Good luck.
One way to have your cake and eat it too would be to put a large space between the two components -- raise the McCormack. For example, you could straddle the sub with a small, two shelf bookshelf or a rack and put the McCormack on top of that. Good luck.