If you are not happy with your vocals, my first concern would be the mic EQ (as Gary mentioned), secondly what mic you are using and thirdly, how you are using it.

The PAS is about as feedback resistant as any system I have used, but you still have to be careful not to point the mic directly at the L1s. Bose suggests you work VERY close to your mic.

I have been using Shure, EV and Crown mics without a hitch.

Eddie