Please explain gain

Gain is a measure of the difference in amplitude ( " volume " ) between the input and the output signal of an amplifier. Not all amps have the same gain, that is, they don't all amplify as much.

When amplifying with two boxes, you want to process the signal the same way with each one. If not, you deform the final output--the highs are boosted with respect to the lows, for example, if your treble amp has more gain than the bass amp.

A competent technician can adjust one amp so that it has the same gain as the other. You have to decide if your electronics' quality makes it worth two expenses here: the modification itself, and the probable loss of resale value in the modded unit(s).

I think you'll find that a lot of line-level crossovers (which is what you need for "active" bi-amping) will have gain controls to provide the adjustments you need. You might also be able to simply put a stepped attenuator in line to adjust gain on one amp.

Gain is a measure of the output signal divided by the input signal. It can be a ratio of output/input voltage, current, or power, depending upon the application.

If the gains are not close when used for bi-amping, then the tweeter and midrange/woofer volume levels will vary relative to each other as the overall volume changes. I have never tried this but I would think it would be somewhat disconcerting to listen to. It should be OK if they are at least close.

I am also interested in bi-amping and found this topic and responses very informative,Thanks.
George