Josh,
I have a Sony condenser mic similiar to yours. I'm not sure of the model off hand but I have one other suggestion. Check to see if your Korg D4 has a phantom power setting. I have a Boss BR-900 and any CONDENSER mic I run with it I have to turn the phantom power on. Otherwise I will get NO signal at all. I went through the same thing and it can be discouraging. Larry's suggestions are also very good! Another great vocal mic for the price is the RODE NT-1. It will be slightly more than the SM-57 and 58 but it will be much warmer on your recordings for vocals. I picked mine up used for $125. Hope this helps! Good Luck! Nate