Pioneer posted this on it's Canadian Web Site:
Poor wiring connections, bad wires or a bad speaker may cause this.
As you increase the volume, you will increase the amount of current you
are drawing from the receiver. To troubleshoot this issue, try the
Disconnect each speaker wire connection. Inspect for any frayed or damaged wiring and replace it.
Check the impedance of the speakers; make sure they are properly
rated for your stereo. In most cases, they need to be 8 ohms. Only a
few Pioneer receivers will support lower impedance.
If the problem still continues, try these steps:
Turn the receiver off.
Remove one speaker.
Turn the receiver on.
Increase the volume.
If the receiver overloads, repeat steps 1-4 (selecting a different
speaker each time). If you disconnect a certain speaker and the
receiver no longer overloads, have this speaker checked at a service
center. It could have a bad speaker component, like a tweeter,
sub-woofer, or even a bad crossover.