Depending on what library you’re using, check high mid freqs as well. Sometimes these need taming a little, which can allow the airy top end to open up a little. Combined with some long blackhole like verb, works well for me.
You could try this free plugin called Roth-AIR. As you may guess from the name it's designed to add air. Even has a big button with 'Air' on it in the middle of the interface. I've used it a fair few times.
I've always put a high pass filter on live background vocals to keep them from being too heavy. I wonder how this would work with a boys choir, maybe just creative eq before any added reverb/ambience would work