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.