Ian, I believe the vocals are the most important part in that you can lose an audience very quickly with an off note or 2. There are at least 10 local bluegrass bands and only one or two have good lead vocals, but.. good vocalist are out there.
Instead of hiring, check out some local talent. If you find a good vocalist to do the demo it helps them and you and saved money too. You might have to go through a few but it might be worth the trouble. Some churches have some very good talent that won't be found anywhere but there in church.
I thought "Diamond Mine" was interesting but his vocals were swallowed up by the instruments. Now I have a bit of a hearing loss so maybe it was me. I'm just saying what I heard.
The other thing we all need and I at the head of the line is objective criticisms and advice because we hear what we want to hear sometimes. I think my wifes voice is great but I would like to know what the truth is about the quality of her voice. Is it good enough for demos or not, but who do you ask?
Just a few thoughts on a warm Sunday morning . Don't know if it's worth the electrons or not. Wyndham