Power: yes, less for just mids and highs, however, consider you might want to try just the tube amp for all, then, if more power is affordable, ...
smaller means less heat, cost, and more placement options.
Crossover does not equalize gain, just distributes these frequencies to this driver ...
You need gain controls on at least one amp (preferably both), get them volume matched to each other at your favorite listening volume, then after that fundamental match, the preamp controls the volume to both amps.