Al, that’s what I thought originally as well, but I was correct by erik squires. It’s not half impedance, it’s actually quarter impedance.
If you look at Parasound A21 specs:
250 watts x 2 into 8 ohm
400 watts x 2 into 4 ohm
750 watts x 1 into 8 ohm (bridged)
If each channel of the amp only saw half the load in bridged mode, it would want to push 400 watts, not 750 watts.