Trouble is Rabbi-Sol , to ask a question like this you have reached a degree of uncertainty in what you are doing.
Mellow voices indeed !! - a complete smoke screen !!!
All you have to do, is, at your next gathering ask your attendees to send you an Email listing 10 or 15 performers they would most like to see at your Events .
Cut and paste them all into Excel, Sort, and see who comes out on top etc.
Tell the performers who you don't want to "book" that, THAT'S how you do it - you're doing it buy some sort of common consent.
It will also tell you what tastes your audience REALLY has, and what sort of music will make them turn up time and time again.
As for mellow voices - I wouldn't say Vin Gartbutt has ever had a mellow voice but he is the No.1. who people come to see in the UK.
They come for the overall performance, it's delivery,content and quality . They want to feel good, and feel they've been given what they are looking for in a performance, and , we all have different expectancies.
I know loads of people with lovely mellow voices, but can't hack (say) 2 x 1 hour sets.
A mellow voice , singing material I don't particularly like , or introduced and delivered by a total introvert wouldn't suit me, but it may well be acceptable to your audience.
If I wanted to hear a mellow voice singing good material , accompanied by his own musical mastery and delivered in a good open friendly manner - look no further than Bob Fox.
Some may differ and have other suggestions, because it IS a subjective matter e.g. I don't want to have an evening of Sea Songs / Shanties by the same person.
Just tell people who pester you, " We canvas opinion from our audience to find out which people who they will pay to see , and unfortunatley you're not on the list. "I" (you)"hope things might change in the future , but that's the way things are right now".
Thankyou for your interest in our gatherings.
Finally ( and with advance apologies to "Cool Beans" ) I have never understood how Tony Bennett,Jack Jones and the like have made a mega-bucks-living out of so-called singing ( mellow voices ?).
I wouldn't pay to see either of them even if they were in your living room.   

Cheers
Betsy.