I have two LFE's attached to my sectional couch being powered by the Buttkicker 1000 watt amp. I thought that the rattle that I was experiencing was from the leg plates but it turned out that there needs to be weight directly above the shakers. My solution(not sure if it will work for you) was to rotate the leg plates so the LFE units were under the couch, not behind it. This cured all rattles. I also considered attaching the LFE's directly to the couch frame but I was worried that the shakers would destroy the integrity of my couch frame. These things "hit" HARD!!
Congratulations on your new addition BTW!
"The bitterness of poor quality remains long after the sweetness of low price is forgotten"