[chuyler1]

Assuming for a moment that the enclosure is sealed, most subwoofers play a bit lower with more air space. Manufacturers typically provide minimum recommended volume and those "in the know" often increase volume to get the subwoofer to play more efficiently in the lowest octave.

Now if the enclosure is vented, everything changes. Port tuning is tied directly to box volume and a change of .3cuft could change the frequency response significantly.

 

[chuyler1]

It is a ported enclosure. I know if it was sealed I could easily just take up a little space inside and be fine. I don’t know anything about ported enclosures so I didn’t know if it would mess up the tuning

I think your best bet then is to model the enclosure in WinISD or similar program. Assume the volume provided by manufacturer is correct (or measure it yourself by filling with packing peanuts then removing and measuring their displacement, measure the port length and cross-section, then input the subwoofer you plan to use. From there, model the box again with a smaller volume but port length and cross-section the same. That should give you an idea of what the difference will be if you were to add some blocks inside.