Is equal gain OK for bi-amping?

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.