They reduced or killed attack, so much less volume. Much different than moongel or something that allows the head to move freely, but slow at a much more rapid rate.In over 40 years of drumming, scouring drum and music magazines and catalogs, going to clinics, talking with other drummers, etc.....I've never heard that internal dampeners were for volume control.
If they were/are, indeed, for volume control then they truly are useless as they don't control volume at all. I can't speak for other drummers but the velocity at which I deliver the mallets into the membrane is what determines volume for me.
Agree with Larry’s post. There’s just no need for them. You can still order some Gretsch drums with them though oddly enough.