This might sound stupid as, but I want to purchase a pair of Sennheiser HD650's and mod them to be closed. Reason being is I want them to be slightly more portable, and leak as little sound as possible. The reason I've chosen 650's is that I've heard the bass is nice and punchy and are built with a quality design and its easy for the grill to be taken off and placed back. I would just purchase Denon AH-D2000's but don't trust their quality to last how and when id be using them.
My thoughts are to use somthing like Butyl Flashing Tape, Dynamat 10415, or some form of putty etc on the inside of the grill in somewhat thick layers to keep as much of the noise in as possible.
I know a lot of you would think this is a terrible idea, but has anyone got any ideas of what material I could use that would work well, or other ways I could trap the sound?
Thanks again, and sorry if I've offended any audiophiles with my idea