Jack, at a first glance you seem to have a mish-mash of gear.
Your singer might not be explaining themselves correctly. Maybe you're sound is the problem for them, not your volume. Is your 2x8 is a combo? How does that work? And, because it's higher, maybe that's all the singer can hear. Maybe these 2x8's at volume might don't sound as good out front as you think.
Why do you need the 2 cabs? Is it for tone, volume or cos they're there?
Like the others I think you're problem is the EQ but, assuming it's loud enough, I'd just take the 115 on the next gig. Get a good sound out of that and evaluate where you go next. I've used different sized speakers to great effect but maybe this combination is just one of those that doesn't work.
If you can't make this rig work I'd just sell both cabs and get a good 212 to go with the amp.