It's not optimal, but it's far from the worst thing you could do. A bowl of water can help give you a visual impression of how much vibration is present on the top surface of your sub. It may not be a lot. You'll have plenty of options on how to decouple the sub from the Harbeth. I would start with rubber/cork HVAC pads. They are a few dollars at most hardware stores, or you could go audiophile approved expensive.
Mijostyn made a good suggestion.