I agree with freediver: Try Auralex SubDudes underneath your Proacs. They will eliminate mechanical transmission of vibrations from speaker to floor. You may still have acoustic (air-borne) vibration issues, which would have to be addressed separately, but eliminating the direct mechanical transmission of vibrations may well be all the improvement you need.
The SubDudes are based on the Gramma pad, which many of my prosound (bass guitarist) customers use whenever they are on a hollow stage, and it works great. The Gramma was invented by award-winning acoustician Jeff Hedback, back when he worked for Auralex.
You might also try stuffing open-cell foam into the ports. It should be a foam that you can breathe through. You can play around with different stuffing densities. This will reduce the bass output, which might be too much for your room, but try the SubDudes first.
(I happen to also be the guy who manufactures a four-sub system called The Swarm, mentioned above, but my suggestion is to try the SubDudes and foam-in-the-ports first.)
Duke