Try playing a constant sinenote (like found on test CDs) of 50/60 Hz at fairly loud volume, and take a AC volt meter to the wires.

As an example, to produce 22W (about what many modern HUs do (RMS, not the inflated peak numbers given in marketing)) into 4 Ohm, you need a RMS voltage of:

U=sqrt(PxR) (because P=U^2/R ) in this case we have P=22W and R=4Ohm, so U=9.38V (RMS)



/Michael