I have been enjoying these headphones today. I had a somewhat thin layer of mineral wool but thickened it up some. I bet I get a lot of the Sigma sound.
I might have put too much wear and tear on the drivers. One seems to have slightly more high treble. The good news is I bought a 2nd hand pair of 407's
today. I think I will enjoy the 407's the way they are for a good while but there is a real good chance they are going to turn into 407/Sigmas. I will take more
time to make the pieces as nice as possible and can hopefully avoid a lot of mistakes. Thinking of using a folded piece of 1/4in aluminum to the angle of the
driver to the earpad then the cage around. That would be about as stiff as it could be. It would be harder because everything did work out so nicely using
a mix with the old Lambda housing.