I believe the ACS (and maybe others?) does the latter, especially as there's absolutely no evidence of a boost in low-end frequency when I use them, unlike previous 'off the shelf' plugs I've tried. Here's a pic I pinched off the internet which shows them going around the corner, so-to-speak.
What I do know is that when I wear them, I can enjoy the gig as if I'm not wearing them. And when I take them out, my hearing is fresh and sparkly like I've been sitting in a quiet room for two hours.
,I'm not an audiologist and I'm happy to be educated otherwise, but if someone (i.e. the specialist, not the OP) is trying to tell me they literally don't work, they're going to have to put a hell of a case forward.