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.
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.