More details & hopefully I won't muddy the discussion:
The lead singer jumps all over the place, has an almost strident soprano voice and usually gets some feedback every night. Exciting to watch, but I feel pity for her poor mic. She's definitely getting supercardioids suggested starting with a Beta 58(A) and a Beta 87(A).
Their third singer is a heartier alto but only has two songs in their songbook. So far it's sounding like their SM58's work just fine with her as long as she doesn't use the lead singer's EQ.
The second singer is the one I'm trying to come up with ideas for. She's a warm, bright alto who doesn't move much or eat the mic. Quintessentially demure. Like her personality, her voice and singing are careful and considered (if that makes any sense).
FWIW, she plays the flute on some songs so a mic that would do double-duty would be a plus, but really the first thing they need to do is get the vocal micing worked out. When they get to auditioning mics, I'll try to get each singer to try most-to-all of them, of course. I know they're aware they never sound as good live as they could (should) but I think they've just never really gotten into the details because 'it's too techy' or the idea of spending $700+ on new mics seems like too much.
The other mic I love is the Neumann KMS105 its a great mic. As for feedback it might be better to have a good sound tech "ring out" the monitors before you start. If you are using the house sound tech. Try explaining to him that feedback has been a problem and that your singer likes to move around. A good house tech will not have any problem making sure feedback is not an issue. But no monitor will withstand the mic in the horn trick at 5 inches, unless its turned off. Most problems with feedback are not solved with a mic. They are solved with eq and proper monitor placement based on the monitor being used, and realist requests for level. What do I mean about that? Well if your guitar player is on 11 and your bass player is on 14 then you need to get everyone to turn down. I often tell bands I work with this.
1-When you listen to your stage sound it should sound as close to the record as possible if it does not something needs to change.
2- The louder you are the less control I have out front and you will be more likely to need hearing aids when you get older.
3- The best mix is one that is not making your ears bleed. The lower the volume the better the fidelity on stage and off, and the less likely you are to piss off the audience. You will sell more tickets in the future because of better sound. With today's PA systems clarity can be obtained with out huge volumes unfortunately there has been more advancement on PA out front then in stage monitor systems. Sure we have in ear monitors but they can be just as bad as wedges if you are not using limiters on them as protection and monitoring how loud the signal actually going to your ears. Remember you have one set of ears that have to get you through your whole career as a musician.
4- Always have the sound guy come up on stage when your playing so he can hear what your hearing that way if you are having a problem like you cant hear your self. He can tell what might be causing the problem and fix it. Remember you cant fix what you cant hear. The sound guy out front is in front of the PA not on stage he cant hear what you hear. Unless you have a separate monitor guy in that case he better listen to the stage
I always got on stage before I started my sound check out front I would always get the monitors working correctly for the band first. Walk on stage and listen to them with the band. It makes them feel more at ease and it also gives me a heads up as to how loud things are on stage and how they sound.
Monitors are an aid to hearing your music not a crutch. The louder they are the more difficult it is to get a great out front sound. If your singer is having problems with feedback it could be the following issues.
1- She is not a loud singer.
2- Your sound tech is clueless
3- Your gear is faulty bad eq ect.
4- Your placement of the monitors is all wrong.
5- You dont take the time to get the monitors eq'ed properly before your show. Hint when the sound guy is ringing out the monitors its a good idea to have ear plugs in or be off the stage, a good burst of feedback can ruin your hearing for the night. And make it very hard to play properly.
6- Your vocal mic is "budget" Get something standard like a sm58 for starters.
7- Your expectations for monitors is way to high. Remember most clubs have crap monitors that are simply so far away from flat it would take a Meyer sim system and a 16 band parametric to make them right. So go through the list see where the problem lies and try these solutions before you go and get a fancy vocal mic. I have been using sm58s for years and never had an issue getting them freaking loud in a monitor system or in the house.