Acoustical foam/padding in guitar cabs...

...do you use any? Why or why not? If you've installed or removed any (so that you have an experience of the same cab with and without it), what differences did you notice?

I've never noticed any significant difference with or without. I've added and removed it in a 4x12 and a 1x12 Thiele-style.

I believe in theory it is used to prevent standing-waves in the cabinet. I don't think it absorbs much acoustic energy at the standing-wave frequencies.

YMMV

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