By line level, if you are referring to the RCA type inputs on a pre-amp, a simple voltage divider will work, though you could go purchase a specially built and packaged device to do this. The inputs are normally rated at 500-1000 mVrms. If you are driving it with an amplifier with a 50W into 8ohm output, you would have 20Vrms. To reduce that to 1Vrms, you could build a divide by 20 resistor network that contains just a series resistor of maybe 1k and a parallel resistor of around 20 ohms or so (not quite /20, but close enough). You should still determine the power requirements of the resistors.