I recently made a big decision to take a leap of faith and experiment with my treasured M50's. Although it turned out to be a much bigger project than I first thought, I'm completely glad I did it.
I've always wanted a good open headphone after I bought the M50 for the soundstage and transparency. So I thought I would do an experiment and make my M50's an open headphone. Of course, I didn't do this blindly, I first tested how the m50's sounded with no fiberloft and no backing, just the drivers and the pads, and found that it sounded much airier and the soundstage dramatically increased. Suprisingly, the bass took on a softer and much more articulate sound to it but didn't decrease in quantity. I also used a Canare quad and wired separate grounds to remove all crosstalk from the M50.
I started by cutting a hole in the back of the m50's, about in inch in diameter. I also removed all the fiberloft and a plastic piece that was over the driver. I put everything back together and really liked the sound, but I heard a bit of echoing in the mids and highs. I decided to use the fiberloft that was in the M50 before and glue it around the driver to dampen the sound a bit and remove the echo in the mids and highs. It worked, and it also made the vocals and highs a lot warmer to my ears. I read somewhere that removing the foam from the center hole of the driver made the M50's more musical, so I did that as well, although I didn't hear any noticeable difference. Lastly I used the Canare wire and used two ground for the left and right which removed all crosstalk and overall highlighted the soundstage. Than I just took a cover off an old speaker and used it to cover the hole in the back of the M50's. Here's the finished headphones:
I am completely impressed with the sound. Overall, I see an improvement in the sound, this includes the bass and mids and soundstage mostly. The bass still has the M50 signature, but is a bit softer, warmer, and has a more articulate and quick punch to it. Although I do think the bass lost a bit detail, I think I like the new bass better. The mids also got warmer, but they also got airier, and much more forward. In sounds that previously were unimpressing in the vocals are much more forward and present. The highs feel much more airier as well, they did sound a bit fatiguing and sibilant at first, but I think i've adjusted to them more. The soundstage has become much wider and deeper. I'm noticing depth in songs i've never before, and it is the biggest improvement above the mids.