He is referring to the output of the Sonos and not the line inputs. But you can adjust the input levels to compensate for the exact problem is trying to fix.

You could try these: [Link: parts-express.com]

Look, you get what you pay for regarding the RTI piece. The reason they created it was for "budget" installations and it has budget features. I am a RTI guy but hate that they claim "committed to custom." Both the AD4 and AD8 do not have all of the features they should to truely be a custom piece. My thought would be to create a piece of equipment that even Crestron or AMX would want and not something I can find with every other mfg. Zektor is the perfect example.