With consumer grade line level outputs, you're more likely to have too little volume than too much. Line outs expect to see a 10,000 to 50,000 ohm load; not 600. Many 'car stereo' systems' speaker level outputs can safely drive aviation type 600 inputs simply by turning down the volume on the stereo.
Best to just try your source of choice, and if level is low, you can add an inline booster preamp. Choose the right one, and it can double as a 'summing amp' to allow additional audio sources, like stall warnings, etc.