I mean, speakers themselves are a much major source of vibration than the D1. I don't know if you meant that you place your speakers on the same desk alongside the D1 or if your speaker is floorstanding. If they are on the same desk, the speakers, when playing, are pushing all the mechanical vibration via the table onto the D1.
I don't know the physical mechanics as to why spike works on the D1 so I won't try to provide a theory here because it is meaningless. Some amount of googling may help here.
I believe that increasing the mass/density of the D1 will help. So will placing the D1 on a separate rack. But the main purpose of using spikes is to curtail the vibration from within the D1 that I and not to isolate it from external vibration because placing the D1 somewhere else is a much cheaper solution.
I do disagree that spikes will not do well on hard surface. When used on carpets or hard ground, people will tend to use the spike alongside spike feets. Also, people who place spikes on their CD players or Amplifiers place their components on 'audiophile racks' (I don't know if they really make any difference compared to normal racks) which are hard surfaces as well.
Anyway, I am derailing. =)