Yes, assuming the external crossover is a bass management controller, i.e., has both a low pass filter whose output connects to the subwoofer and a high pass filter whose output connects to the main speakers via their amps. The connections will be exactly the same whether or not the filters are passive or active.
I seriously would consider the potential for problems with a passive unit. An active one should have relatively constant input and output impedances and the outputs will be driving the loads not your preamp. Their should be no impedance matching problems with an active crossover and any of the solid state amps (including the sub's). You'll have to be aware of impedance matching between the preamp and the active crossover. You need to know the worst case output impedance of the preamp not the typical value at 1 kHz.
With a passive unit, the effectiveness of the filters will depend on the impedance interaction they experience with their source and load impedances. Their load impedance, the input impedance of the amps, should be relatively constant and not a problem. The filter can be built with that knowledge. But, their source impedance, the output impedance of the preamp, will likely vary with frequency -- increasing in the bass region. The filter can be built knowing the worst case impedance of the preamp, but it will still have an issue with the impedance changing.
There is also the insertion loss of the passive crossover.
Ultimately, the variation of the output imedance of the tube preamp has to be taken into account. It is less of an issue with an active crossover.