So if your AVR's AMP is lower in power output per channel (stereo) than you would like, passive bi-amping will actually make that worse?
Usually, but it depends. If the power supply is the limitation, which it usually is in an AVR, then you've made the problem worse by adding additional loading on the supply. Voltage output is unchanged, but the current output from each amplifier is lower than if a single amplifier is used, so if the amps are current-limited then it could help. However, the vast majority of audio amplifiers are designed to be as near as possible to a pure voltage source, so "passive" bi-amping the AVR way is unhelpful. You are also increasing the heat generated by the AVR, which has other negative effects like shortening component life. If you really need a bigger amplifier, then buy a bigger amplifier, or implement conventional bi-amping with a crossover before the amplifiers and size them accordingly.