I appreciate Slv's clear definition of gain. Mine is less precise. Although I tried to simplify for a first-timer, I am not highly knowledgeable technically, and to simplify well, you have to have mastered something sophisticated first.
I think it worth mentioning that you can biamp actively, with an additional powered crossover box between the preamp and the amplifiers, or passively, using the speakers' own crossover. If your speakers are already set up for biwiring, they can be passively biamplified. (And of course, there is a possible upgrade path in going from passive to active, as promoted, most notably, by Linn.)
I would be wary of an active crossover with built-in gain controls. My hesitation has to do with the base line reference for the amplification. There is no way I can see for the crossover to know the amount of gain each amp is applying, so all adjustments have to be done by ear. The result could be good or bad. I would much prefer to know that each amp was producing signals at the same level before going to the crossover.
Also, the additional volume controls make for more signal processing, which is potentially degrading to the sound, as well as extra expense.
I would want to be sure these questions were answered before using an active crossover with gain controls to compensate for two dissimilar amps.
