I just got a Yamaha A-S700 integrated Amplifier. I have yet to hook it up (new Polk RTi A7 speakers just arrived yesterday). In reading the Yamaha instructions I found a "BI" hookup for the speaker listed in the instructions. This was the first I had heard of this. It is where you separate the speaker's tweeter/midrange from the woofer and drive each with its own amplifier. The instructions tell you how to do it. You remove the shunting bar on the speaker and run separate wires to each part of speaker for a side (left or right). So Speaker "A" out drives the tweeter/midrange and Speaker "B" out drives the woofer. Sounds good!
But there's a rub. I found the schematic on the internet as part of the Service Manual and there are two amplifiers, left and right. The Speaker A and B are connected to a switch from the amp for a side which make the BI connection meaningless. The removed speaker shorting bar is replaced inside the amp! The A and B speaker terminals are for local and remote speaker control: A on, B on, both A and B on, or both off. There can be no improvement in performance from a BI connection to the speakers on this amp.
I think it a little disingenuous for Yamaha to try to fool users into thinking they are doing something special when they hook up the speakers BI when it only allows the tweeter/ midrange and the woofer to be turned off and on separately. That's not even a usable idea.
In a true BI connection I think nothing would be gained unless you limit the bandwidth of each of the amps so as to minimize harmonic distortion (??). A low pass filter before amplification for the woofer and a high pass filter for the input to the tweeter/midrange amp. That is, crossover before amplification so as to minimize the frequencies in each amp. I see no advantage to BI if both are full bandwidth except you get full power to each part of the speaker rather than half power to each section of the speaker (??). That has little effect on the sound quality, I would think.
Did I miss something?