This is going to sound like a really dumb suggestion, but have you flipped the mic around 180 degrees to the other direction? The symptoms you are describing also sound like the mic could be facing the wrong way, or that you are using an omni (all the room noise at high gain). I know that the KSM-27 is not an omni, so you might try singing into the wrong side. Practically everyone does this at some point or another, myself included.