Without seeing some type of schematic, or layout, having a "zoned" outdoor system with multiple volume controls just sounds overkill to me.
That said, I think you need a multi-channel amp.
I'm running my whole house (3 zones, master z1, family rm z2, sunroom z3)audio off of an Emo A-800. 3 pairs of speakers...the Pre/pro only has 3 zones with control...main, Z1 & Z2...so to get volume control over Z3 I wired Z2 & Z3 into a on/off, volume control box which is probably similar to your Niles unit. All 6 speakers are powered by the A800.
https://emotiva.com/collections/basx-series/products/a-800