I understand that when you add polyfill to an enclosure, the sub works as it is in a larger enclosure. But if your enclosure is already at the maximum recommended volume by the manufacturer, doesn't that damage the sub? also do you need and if yes, how to you take into account the polyfill when tuning ported enclosures?

 

It does not work as if it is in a larger enclosure. Being in a larger enclosure would almost never damage the driver anyway. A good box simulation program will let you very the amount of fill used in the simulation. Do not build a ported enclosure unless you have simulated it first in (e.g.) WinISD or you are 100% sure the design does not suck. Ported enclosures are EASY to screw up very badly, and even major manufacturers can get their published designs wrong.