Using a Pre-Out to Bi-amp

This will work if the input sensitivity of the amps in the same. Otherwise you will need something to balance them.

We are both saying the same thing. If you do this you will have to make sure that the sensitivity (or gain , same thing) of the amp you are buying is the same OR HIGHER than that of your integrated amp. Why? Because the output of your integrated feeds directly into your power amp , if it is more sensitive (more gain) than your external amp there will be no way to balance their gain. The ideal solution would for them to be exactly the same but I wouldn't chance it, manufactures specifications are often inaccurate. I would buy an amp with considerably higher gain (more sensitive) because then I could even them out with a volume control in the input line. A simple one is easily constructed or get one at Radio Shack or more upscale source.

The input sensitive is the amount of current required for the amp to reach full output. For example one amp might require one volt , another two volts. This is the governing factor in the choice of amps in this case I would think. Input sensitivity is always specified for a power amp , I am less familiar with voltage gain. I would assume that it is the mirror image of sensitivity, describing how much the initial preamp signal is amplified. But it is not necessarily the case that you should have equal output from both amps. The tweeter or tweeter/midrange will require much less power than the woofer so having the same output from both amps might overdrive the high end. In the past when amplifiers were offered with several amps on the same chassis the treble amps were always of considerably lower wattage than the bass amp .I think we are saying the same thing with different terminology. The critical factor is the bass amp having greater sensitivity
( higher voltage gain) so that it can be adjusted by a volume control as we have both suggested.

It's not often that you see a post that is totally wrong but Elevick has achieved that distinction. The time difference will be totally insignificant and completely inaudible. If you look through equipment reviews and forums you will see that the majority of bi amp systems use amps that are not identical.