Who uses an omni for live use these days? Even in the studio, it's not very usual to use an omni for miking a single singer...
And live, it's the kiss of death
The problem...most wireless headsets are omnidirectional....designed for theatrical stage use, allowing the mic to pick up all stage voices...That is why I still do not use a wireless headset mic...and it is compounded , destroying any harmonizer use too...
When I find a good unidirectional mic with a nice tight pattern , I use it...The Samson G7 turned out to be a great mic for me..