Try a 'constrained layer' dampening.
The idea is to have the TT with an increased mass on top of a rubber, ? whatever, rubbery like layer, then another massive layer. So the rubbery layer really cannot move, it can only absorb vibrations.
i use round 3" thick concrete patio blocks (5" round) on my basically cheap TV level rack, with rubber chemical bottle stoppers, then the 2 TTs with mass added to the bases under the table. I have the TTs across the room from my Maggies 3.6 and do not have a problem, though the floor is wood.
Also, even if your rack is vertically stuff, it may not be horizontally stiff enough. See if pushing on the top of the rack sideways while the phenomenon occures lessens the woofer pumping. If it does, then your rack is moving sideways!
The idea is to have the TT with an increased mass on top of a rubber, ? whatever, rubbery like layer, then another massive layer. So the rubbery layer really cannot move, it can only absorb vibrations.
i use round 3" thick concrete patio blocks (5" round) on my basically cheap TV level rack, with rubber chemical bottle stoppers, then the 2 TTs with mass added to the bases under the table. I have the TTs across the room from my Maggies 3.6 and do not have a problem, though the floor is wood.
Also, even if your rack is vertically stuff, it may not be horizontally stiff enough. See if pushing on the top of the rack sideways while the phenomenon occures lessens the woofer pumping. If it does, then your rack is moving sideways!