Steve,
my official answer is "I don't know." I'm confident this answer will hold up in court should your tonearm explode.
My half-wild guess: some arms have damping troughs that are filled with silicone and those arms can be adjusted by changing the amount of silicone in the trough. I checked the ASL site for info on the Spacearm, and Nottingham's Tom Fletcher was quite proud that his arm DIDN'T use silicone damping, but it was unclear just how damping would be adjusted with his non-silicone system. His system seems to rely on stabilizer bars for his unipivot design, but again I can't tell how damping is adjustable.
I too have a Spacedeck, but with a Rega arm and a different cartridge probably. I HAVE had trouble with resonance feeding back into the cart from the speakers and "solved" this by physically moving the TT. The TT must have been sitting in a bass node or something where the arm/cart resonance was especially excited (but my resonance was much lower than yours). This is not really a solution, but it's just avoiding the problem.
Since the resonance of the arm/cart is dependent on the cartridge used, could you swap carts and see how the resonance changes?? Again, not a good solution, but maybe better than your situation now. Also, a tonearm's resonance can be changed by wrapping the arm with something like a Sumiko analogue survival kit or Atma-Sphere's equivalent (you could try Teflon tape too). This should CHANGE the resonance, but not necessarily make the situation better.
Sorry that I haven't come up with a better answer, but it's the best I can offer.
