Koa isn't a bright spruce tone, nor the mellower mahogany, knowing this helps to balance out tones someone is looking for.
I'm assuming you're thinking about Nylgut too...
I've played all the Aquila strings on a Kanilea, and the Nylgut were the warmest tone.
It's always been my understanding with Nylgut, they are just a warmer string, no matter what Uke you play them on, like Nylon strings in general, like D'addario's Nyltech.
Actually for a bit more brightness and punch, for someone not wanting to go Flurocarbon, the Nyltech might be the way to go.
Compared to around 12-15 other sets I've played, the most commonly used ones out there you find most Uke players using, Worth, Martin, D'addario, etc., I wouldn't call any of the Aquilas, or the Nlygut better or worse, just a matter of taste and styles...
Food for thought, the Nlygut is the favorite of Kanilea, Joe the owner himself will tell you, it's the reason Kanileas come stock with them, because they think it gives their Ukes the best tone.