No, the Macintosh Quadra 800 does not use a true line level input, but rather one designed for the original (unamplified) Apple microphone.
For a true line level input you need an input level attenuator to avoid overloading the input amplifier stage. Here are the input level specifications:
- Sound In port sensitivity: 1.15mVrms for full scale output
- Sound In port clipping level: 150mVrms
- Sound In port overload level: 2500mVrms