Is equal gain OK for bi-amping?

If the amps have 2 very different input impedances, say 20K and 60K and they have equal gain, won't the output volume be different also because of the impedance being un-equal?

No.

If the two preamp outputs are not separately buffered, the inputs of the two amps will see the same input voltages at all times, because they will be driven from a point that is the same from an electrical standpoint.

If the two preamp outputs are separately buffered, and both power amps have much higher input impedances than the preamp output impedance (which is the relationship that should be provided anyway, in any preamp-to-power amp interface), the difference in power amp input impedances will not result in a significant difference between the voltages that are input to the two amps. "Much higher" in this context would mean perhaps 20x or more, relative to the worst case (maximum) preamp output impedance across the audible spectrum.

Best regards,
-- Al

Before we start on amps, what are the speakers? If it's a 2-way, chances are biamping is a waste of time because the crossover is too high to be of any benefit. Even on a 3-way, anything over 300 Hz, is getting uncomfortably into the sensitive midrange. Personally, I wouldn't biamp over 150 Hz unless going active.

There are Endler EVS/Rothwell inline attenuators but they aren't designed for fine tuning as much as coarse volume control and the input impedance can be low for an already divided setup.

Ngjockey,
IMHO biamping can only be ideal with two identical amps. Other than that I'd go active.

From a theoretical standpoint, "passive biamping" is really only advantageous when using a pair of identical, direct-coupled solid-state amplifiers that have a low output impedance, especially when they're Class AB or B with conventional unregulated power supplies. In this case, the gains are matched precisely, and separating the high-current demands of the bass can significantly improve the linearity of the mid/high amp. These types of amplifiers also don't exhibit odd side-effects from a high-impedance load for a good chunk of the signal spectrum with which they're driven.

With your particular setup, the Class D bass amp could possibly have an increase in its ultrasonic noise output, as its output filter is load-dependent. And many tube amps will misbehave into a high-impedance load; if this occurs at low frequencies it could be prone to LF oscillation (motorboating) depending on its design.

Not that getting good results with such a setup is impossible, but you will definately want at least one of the amps to have a variable input sensitivity for fine-tuning, and accept that you have many unpredictable "tone-control" interactions going on that will make the performance rather quixotic and unpredictable.