The spec in the manual says 55 ohm output impedance. (not 55k)

So does that mean if I build an interconnect with a 55 ohm resistor in
the tip, the signal will be reduced 3 db??

No, that's the output impedance of the head unit. What you need is the
input impedance of the amplifier. Oh yeah, and you may want to reduce by
more than 3dB.

Oh yeah, the other way to do it is to use 2 resistors and create a voltage
divider.

Really, you don't need to know the input impedance. All it does for you is
give you a good starting point. If you can't find it out, just assume 20k
ohms.