Hi Gospel,
I'm here lurking to learn about recording,but I feel qualified to comment on this.
The 58A differs from the regular SM-58 in two ways.The neo-dymium magnet gives a bit more gain (volume) before feedback.In other words,it has more output than the regular 58 at the same setttings.
It also gives a bit more of a "presence" boost."Presence" is old-timer talk for high end.Shure vocal mics like the SM-58 do this anyway.It's one of the reasons they're so popular--in a murky,muddy live sound environment where the acoustics are poor,the vocals cut through,and are crisp and clear.
The Beta 58 just does it a bit more.
You don't give any specifics about your eq-ing,but I'd hazard a guess that in the same sound environment,you could turn the volume down a little,and possibly the highs just a hair,and you'd get more what you're used to with the regular 58's.This in itself may fix your problem.
Since they have higher output ,the way the mics are "worked" distance-wise probably added to your problem.If each singer is used to singing (for instance) 3 inches from the mic for this part,five inches away for that louder part,et cetera,this would throw your blend off until each singer learned how the new mics react.
I would turn down the gain a little."A-B" a new and old 58 before a rehearsal (or just on your own with the PA),and figure where to set the volumes and eq.And then let the singers get used to working them and blending.
I could definitely see where your blend could be thrown off without adjusting the PA and getting used to the new mics.
I do believe it's worth the trouble.I've been singing for 30-some years,and in my opinion the Beta 58 is a noticeably better sounding mic for live gigs,especially in places that are poor acoustically.
On the other hand,if y'all are singing in the same 2 or 3 places regularly and the acoustics are good,and the singers hate the new mics,then sell 'em on Ebay.
Nothing wrong with a regular 58 or 57 either.
By the way,I prefer the 57 over the Beta for recording.
