Hi I have 2 speakers in kitchen ceiling and 2 outside speakers in an outside space,

They run off the same amp, and I can turn each pair/space off separately as the speakers run through rocker switch, which works perfectly 

How can I alter the volume as kitchen speakers are louder than outside so could I control the kitchen ones through a volume control dial instead of the rocker switch, then I could reduce the volume to the inside ones via the turn dial, if so what make as I can only find one that controls upto 30w which is not enough for the sonos speakers 

The controls are all in a cupboard so doesn’t matter what it looks like 

just that when it’s loud outside it’s defeating in the kitchen 

Any suggestion please