Defoaming is compound specific.  It's as much a black art as science, and specific compounds work better on some foaming problems than others.  The reasons are often too complex and too detailed to explain here. 
The Polydimethylsiloxane defoamer is essentially a hydrophobic absorbing compound.  The hydrophobic absorbing property makes it effective as a defoamer. 

The challenge will be finding the right compound that is effective at the desired rates. 

There are a couple of things I might try.  First is to introduce solids into the tank prior to final rinse.  Something with a high surface area, might help absorb some of the PVA and reduce the foaming potential of the fluid.   I'd start with something like sawdust, and then switch over to something like sodium silicates, as trials.  You might also want to check the pH and see if the pH, especially lower pH has any effect on the ability of the compounds to foam.  I suspect that it might.

I believe that calling in other chemical suppliers might help.  It sounds like one company's compounds might be more effective than another company's compounds.  Contact the local polymer sales representative and have him or her send you some samples, then test it out in the laboratory to confirm results before going full scale.