Hi Clipped,

if your rail voltages are +/- 34.5V and the amplifier has no voltage drop under load you will have a sinus on the load of 34.5Vpeak.

1st method: P=U^2/R

34.5Vpeak are 34.5Vpeak/1.41=24.5Vrms

Your RMS power will be (24.5Vrms)^2/4ohm=149Wrms

2nd method: P=R*I^2

With 34.5Vpeak on a 4ohm load your current will be Ipeak=34.5Vpeak/4ohm=8.62Apeak.

Your RMS current will be 8.62Apeak/1.41=6.11Arms,

thus the RMS power is (6.11Arms)^2*4ohm=149Wrms

3rd method: P=U*I

The RMS power will be Prms=Urms*Irms=24.5Vrms*6.11Arms=149Wrms

As you can see the three methods have the same solution, it is the ohm's law...

Now, if you put the amp in mono (brigded mode) your peak voltage on the load is doubled, so that you will have 69Vpeak.

69Vpeak is 69/1.41=48.9Vrms and then

Prms=(48.9Vrms)^2/4=598W

Putting an amp in brigde will double the voltage on the load and thus multiplies the power by 4. This is truth only in ideal case, normally the output voltage is not doubled because of the increased current demand increase the losses in the output stage and also the power supply may loose some volts. A good amp, designed to work in brigde mode normally multiplies the power be 3 or something like that

ciao