I have removed / added to my old marshall 2x12" sealed back. With little in it it sounds hard, with more it sounds softer and a little dull.
In a ported cab it's a little different. On hi-fi speakers you will hear some of the midrange from a woofer in a two way system beeing softer. Having some damping material on the insdie walls will prevent the midrange from moving and beein reflected and then coming out of the mebrane.
I used a lot of time experimenting with damping materials on ported Hi-FI speakers, however the shape and the wood type of the speaker Cabinet is more important. You want to hear the speaker and not the cabinet resonating. MDF wood is the best material, heavy but using a MDF front plate will improve the sound.
That is not perhaps always so importante in a guitar speaker cabinet, but perhaps a small layer in the bottom of the speaker cabinet is enough for many. I