Hi Glenn.
My 0,02:
Try another mic. Along with many others here, I use EV. Both the N/D 967 and the 767A are excellent mics and vey FB resistant.
Second, if you use reverb, try and use as little as possible. Reverb is no 1 cause for feedback. If you really can sing, you don't need reverb at all. Most rooms give you natural reverb anyway. Good luck!

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Roy-Andrè