Well I did a 100km ride last Friday and no numbness. Last year my Brooks was way too high at the nose. Now its level and I am finding that just right for me. It did take some time to break in as I am a bit of a lightweight. Someone here on BF suggested "helping" the break in process a bit with some gentle, encouraging taps with a ballpeen hammer. It seemed to do the trick. It now has a pair of nice, cute little dimples and should only get better with age. A regular massage with a bit of Lexol and some Snow Seal on the underside once a year or so is all it gets.
Numbness would not be from restricted blood flow. No that would make "willy" turn black and fall off. The numbness would be from the compression of nerves. Just like when your arm or foot falls asleep.