Reduce to line level from 8 ohm, 6 w stereo
simulate this circuit &ndash; Schematic created using CircuitLab

Something like the circuit shown should work.  I'm assuming that the amplifier will be happy driving a 18 ohm load, rather than 8 ohm.  Doubling the resistance halves the power dissipated.  Even so, R1 needs to be something chunky, such as a 2W resistor.

I chose the relative values of R1 and R3 so that you don't have to run the amplifier at full volume.  Turning down the volume a bit significantly reduces the distortion.

If the impedance is important, you'd have to halve the values of both resistors, and use an even bigger resistor for R1, such as 5W rated.