When harmonies "warble" it's often because other instruments or sounds are leaking into the mic. There are several ways to overcome this:
- Sing on the mic as much as possible, even to the end of a phrase
- Reduce the level of other instruments arriving at the mic either by reducing their volume or pointing them away. In the case of bass, the only thing that will work is to reduce the level or play in a bigger room.
- Use dynamic, not condenser mics if you are having this problem. Condenser mics are much more sensitive and will pick up lots of other sounds.
Also, be encouraged that thousands of people are using vocal harmony processors successfully in live and rehearsal situations. It's just a matter of finding what's causing the problem and working around it in the right way.