Minorl, your posts are always extremely knowledgeable and helpful, but in this case I'm not sure that you are correct regarding the need for sensitivities to match.
As I'm sure you realize, sensitivity is the input voltage that is required to drive the particular amplifier to its maximum rated output power, whatever that may be. I don't see how that relates to achieving a suitable match between amps in a passive bi-amp arrangement.
The goal would be that equal input voltages to the two amps result in equal output voltages. Assuming equal load impedances, that simply requires that the voltage gains of the two amps are equal.
What does concern me, though, is that the output voltage of the ss amp will be essentially constant as a function of load impedance, while the output voltage (and hence the gain) of the tube amp (with a higher output impedance) will not be. Depending on the speaker's impedance vs. frequency characteristics, that could result in a slight but conceivably significant deviation from frequency response flatness, especially if the tube amp's output impedance is high (i.e., its damping factor is low) and the speaker impedance at mid and high frequencies is low.
Also, the OP should keep in mind that in a passive bi-amp arrangement, if there is a large disparity in power capability between the two amps, much of the power capability of the ss amp will not be utilizable, because the volume level that can be utilized will be limited by the clipping point of the lower powered amp.
Oops, seem like I forgot input impedance has to be the same also.
As long as the input impedances of both amps are much higher than the output impedance of the preamp, which they should be, that shouldn't matter. If the two outputs of the preamp are not separately buffered (as is usually but not always the case), the combined input impedances of the two amps should be at least 10x the maximum output impedance of the preamp across the audible frequency range. That combined input impedance is equal to the product (multiplication) of the two input impedances, divided by their sum.
Will I really be wasting the power the upper amp makes in bass notes if the speaker's crossover blocks them, doesn't the speakers crossover create a much higher impedance at bass frequencies thereby not alowing the upper amp to waste that energy?
In a passive bi-amp arrangement, the speaker's crossover will dramatically reduce the current demands on the upper frequency amp, but will not change the range of voltages that it needs to output. Its output voltage capability, however, may increase, at least a little bit, as a result of the reduced current demand.