I've been reading that stuffing can increase the apparent enclosure volume to the driver, which result in a lower F3 in a sealed speaker. Does this apply to passive radiator speakers and mean the PRs would be tuned lower than without stuffing?
So stuffing does not take away apparent enclosure volume right?
I'm making a very small speaker with maybe 2.5L of effective internal volume, so every bit of enclosure volume matters.
If it doesn't, I can line the walls with felt, and the stuff with polyfill or something. I'm reading it is not good to overstuff the enclosure, so I'm not quite sure what the balance should be without some trial and error.
Not in small quantities. You need a fair amount to make it appear larger and even more makes it smaller again.
Higher air coupled box modes are not difficult to absorb with moderate damping. If you are going to use limited material it can be better to space it away from the walls. Waves build pressure at the walls and the opposite (velocity ie movement) is needed for this kind of damping to work.