Larry kindly gave me a Yammy HP-1 today so I had a quick play...
As posted on H-F...
So I quickly whipped the HP-1 apart to test a theory, and it worked! Not perfect but a nice proof of concept.
That 3M micropore paper tape we've talked about.. I bought a 2" wide roll of it at the drug store a few days ago. After rubber-gluing the driver to the faceplate to seal it properly, I taped completely over the back of the driver, the whole thing, with micropore, which gives just a slight gap over the center tab wire. Trimmed it flush with the driver edge and put a sheet of thin, medium density felt over the existing foam pad in the cups and screwed it back together.
I cut a donut of the same felt to go over the front of the driver, under the pads. Anyway, end result? lots of really deep bass and lots of treble. Cuts their efficiency by half and is putting quite the strain on the mini3 I have Kabeers YH-1 here as comparison and it's quite the transformation. Deeper, more powerful, more slam and clearer top. They actually have some bite to them now but they still keep most of the smooth midrange
As I said, not perfect but a very quick and simple mod anyone can do in the time it takes to unscrew and screw the cups if you don't bother with the glue stuff.
So yeah, fun little experiment. I might try to redo it using a sheet of leather over the driver instead of the micropore tape which should tame the highs a bit better.