Hi Bibasset - I did not see your post until just recently.
Obviously, setting your amp on your subwoofer is not something we would normally choose to do, but I understand that we don't always have an ideal situation. In fact, the top of the Vandersteen sub is fairly "quiet," so if you have to put it on your sub, this one is better than most. The DNA amps have good damping feet built-in, so this will help to protect you from high-frequency vibration - there is not much you can do about low-frequency energy. (Please be sure to use a thin paper or plastic coaster to protect your subwoofer's finish from the damping feet.) Other types of feet may change the sound so you should feel free to experiment, but they will probably not change the low-frequency coupling. You have several inches of separation between the DNA-0.5 circutry and the subwoofer amplifier, so I don't think you have to worry about electric or magnetic field coupling at all.
Move the amplifier to a dedicated stand if you ever have the opportunity, but if marital bliss demands you leave it on the sub in the meantime, I suggest you defer to the wife ;-)
Best regards,
Steve McCormack
www.SMcAudio.com