Jan,

For (home)studio use you could try recording the vocals dry with your condenser mic. After recording you can send the vocal track to your GXT. Be careful not to drive the GXT too hot, since it is a microphone-level input and you feed it at line-level. Record the 100% wet signal from the GXT to another track. This way you have much more control over your effects/harmonies and you don't have to deal with bleed-issues.