Dave, is LF mechanical energy truly benign, slower oscillations should impact sonics as well, I prefer to drain off all energy maximally. The fluctuations between motor and tt are just another complication. I suspect it might be best to spike motor as well, haven't tried yet.
My take is that VPIs should be best situated with large mass spikes coupled to massive vibration sinks maximally decoupled from external vibrations, ie. wall mounted or cement slab decoupled from rest of flooring.
I will soon be building a huge sandbox (planning on over 300 lbs sand), changing out stock spikes for Star Sound large mass brass spikes, figure out some means to spike motor, all mounted on my de-coupled wall shelf. This should test my theories, based on previous exploration in this direction I expect much better performance.