IMO, you will lose some coherence and focus in the sound of your monitors if they are placed on the subs because the vibrations from the subs will cause the monitor cabinets to vibrate as well, thereby causing the drivers to oscillate up and down and side to side with the vibrations. Speaker designers often go to great lengths to build speakers with extremely dense cabinets to minimize vibration and isolate drivers from external movement (see Magico, and Green Mountain Audio for two examples).
At the very least it'd be smart to place some vibration damping devices between the monitors and the subs...like Aurios bearings, sorbothane footers, or one of the products from Herbies. Perhaps even Aurios placed on top of sorbothane discs (hadn't thought of that 'til now...). Von Schweikert utilizes rubber footers between the upper and lower modules of their VR4 loudspeakers. I have augmented mine with some larger, thicker, and somewhat softer sorbothane discs.
It seems to me it'd be fairly easy to see the efficacy of using a damping device between the monitors and subwoofers by placing a glass of water on top of the monitors with and without the damping devices installed. Play the same track of music each time, and observe the vibration pattern that becomes apparent in the water. Theoretically, there should be less movement visible in the water when the vibration control devices are being used.
FWIW...
