Speaker level to line level
Ultimately you want a voltage, so it will be easier to think in terms of the voltage out of your amp.  You say your amp has "120 W" output.  Presumably that is maximum into a 8 &Omega; load.  Sqrt(120W x 8&Omega;) = 31 V, which is the maximum RMS voltage from your amplifier.  You want to get that down to 1 V or so, so something like this would work:



However, that would require cranking your power amp to maximum gain to get the full line voltage signal.  I'd probably attenuate by only half as much, which means making R1 1.5 k&Omega; or thereabouts.  Whatever the line level signal is fed into isn't going to get hurt by 2 V instead of 1 V, just that it will be very loud and may get clipped.  That would be a clue to turn down the volume going into the power amp.

Keep the power dissipation of R1 in mind.  The voltages accross R1 and R2 will have a ratio of R1/R2.  When 1 V is accross R2 in the example above, then 30 V will be accross R1.  (30V)2/3k&Omega; = 300 mW, so a "1/4 W" resistor isn't enough.