vent size vs. volume

[David Deratany]

Am I correct in thinking that increasing the volume of a cabinet by 1) removing volume consuming acoustic foam, and 2) replacing it with virtual volume increasing fiberglass will require a smaller vent, other things equal?

Or is there more to it than that?

[marcuswilson]

If you are increasing the virtual volume of the box, the port needs to larger for the same tuning.

One way to tune cabinets is to adjust the port size in order that the two impedance peaks are of equal value.
Normally correctly ported cabinets result in two equal value impedance peaks in the low frequency. If you know how to measure the impedance over the range of frequencies of 20 to 100 Hz, this is an easy way to get a good port tuning.