Unrestricted driver/magnet hole results in too much bass and midbass that bleeds everywhere, even becoming boomy. Doesn't turn the HD650 into basshead cans, but it sure throws the whole thing off. On top of that, entirely removing the front foam results in too much highs in my experience. That's why there's the coin mod, you get just enough foam out to get an improvement, not more. Back when I'd gotten mine, I'd tried them without the front foam and they didn't last 5 minutes before going back to having the foam in there and doing the coin mod. Doesn't sound good. I wouldn't be surprised if these HD650s had a V shaped frequency response relative to stock or KISS.