Bastards. We need to occupy D&M Holdings.
So I think Fostex was onto something putting thin porous paper (ish) stuff behind their more recent orthos including the T20v2 and T50rp. I've been playing with coffee filters and they are my new favorite damping material. I'm going to be getting a bunch of different porous papers to try if I can source that somewhere. I'm thinking an online art store or something. But coffee filters do a much better job than the tapes IME. They bring out more treble while bringing out less upper midrange peaks. And IME they do not constrict the soundstage any more than a thin piece of felt would when placed on the back of the driver. I've been using the coffee filter instead of the first layer of felt in the typical damping scheme, so driver-paper-foam-felt-cups. They work so well at bringing out treble that they can actually get my YH3 to have elevated treble without much of an elevated midrange. Which is a first for me. I actually get some sibilance unless I use a donut instead of a disc, so I'm going to try to find some slightly more porous paper.
I found some silk coffee filters online which must be from the 1800's or early 1900's but they are way too porous even when I use like 6 layers. They have a cool old box though.