I misspoke in the above post. The amp's power is constant for each load. If the power is constant and the load changes, then the voltage must change as well for different loads.
For Example:
P=400 watts
I = 4ohm, 2 ohm, 1 ohm
@ 4ohm, V = 40
@ 2ohm, V = 28.28
@ 1ohm, V = 20
The question...