You could use a pair of pontentiometers to lower the Sonos volume on the line level, but it sounds like the other two need to come up as well, in which case, a car audio line driver is your best bet.

I saw a couple of 16db boost with RCA in / out for $14 shipped, but you would also need a small 12 volt power supply. I am sure the amperage draw on those is very low.

Then again, built in gain controls should be standard.