The 24i comes from the factory with the Headphone pot controlling the headphone outs and the main outs always being active with no volume control. IF you/or the customer change an internal jumper you can make the headphone pot control both the Heaphone jack and the main outs. This is potentially a good way to control the level to your speakers.

Addititionally, although the unit is designed for balanced +4 inputs, you can use -10 line levels into it. There is already enough dynamic range/signal to noise to do it, plus they are going to update their software to let you choose between -10 and +4.